Infectious Disease
2.4a Principles of infectious disease
Introduction to the systematic study of disease
The principles of susceptibility to disease and the pathological processes of disease were introduced in Chapters 2.1, 2.2 and 2.3, where it was explained how some of the processes of disease, such as the disorders of excessive clotting and cancer, are considered as diseases in their own right. However, in most diseases, the name of the condition does not necessarily indicate the underlying processes of disease. From this point onwards, the information given about diseases is structured according to either the pathological agent (e.g. infectious diseases), the physiological system affected (e.g. diseases of the cardiovascular system) or the type of patients affected (e.g. child) rather than according to the process of disease.
As practitioners of acupuncture, it is not necessary, of course, to be fully conversant with all the different conventionally described diseases and their treatments. However, when it becomes relevant to the management of a particular clinical case, it is of great value to be practiced in researching information about a disease and its treatments. This approach first requires a basic understanding of the physiology of the various systems of the body and the processes of disease as described in earlier sections. It is also important to be familiar with how information about diseases is presented systematically in medical texts and websites. For this reason, the disease-specific information presented in the following sections is written in language chosen to be accessible to non-medically trained practitioners, but ordered in the way that information is structured in conventional texts. It is intended that these sections of the book will provide a conceptual bridge for the understanding of more in-depth descriptions of disease and therapeutics found in other more technically detailed sources of medical information.
Introduction to the study of infectious disease (communicable diseases)
Diseases can be broadly classified as infectious and non-infectious. According to conventional medicine, infectious diseases result from damage to the body following infection or infestation by an external infectious agent. Infection may lead to a range of processes of disease and may affect one or more of the physiological systems.
The infectious agents are summarized in Table 2.4a-I. They may be microscopic, in which case they are called microbes. If a microbe is a bacterium or a single-celled animal such as an amoeba, the term microorganism may also be used, denoting that the microbe is a life form. Viruses are non-living microbes. Most infectious diseases are caused by microbes, but larger creatures such as worms can also cause infectious diseases. When the infectious agents are arthropods (insects and mites), the term infestation may be used to describe the disease process.
Agent | Description |
Microbes | |
Viruses and prions | Non-living particles capable of replication; prions are made of protein alone, and viruses of a combination of protein and genetic material |
Bacteria | The smallest life form; bacteria consist of single cells, but are more simple in structure than the generalized cell described in Section 1.1b |
Fungi | Yeasts and molds; like bacteria, these are composed of very simple cellular units |
Protozoa | Single-celled animals such as the amoeba; the cells of protozoa contain all the organelles of the simple cell described in Section 1.1b |
Larger organisms | |
Roundworms, flukes and tapeworms | E.g. threadworm |
Mites | E.g. head and body lice |
Insects | E.g. fleas |
The definition of infectious disease
Infectious diseases are so termed because they are contracted by transmission of the infectious agent from a person, animal or even an inanimate object (termed the carrier). This also explains the use of the term communicable.
Not all infections can be classified as infectious diseases. For example, an infection can arise when a person succumbs to microorganisms that naturally live within the body, but which do not usually cause disease in a state of healthy balance. A bladder infection, also called cystitis, is an example of such a disease, in that the bacteria that cause a person to develop cystitis usually originate from the person’s own natural perineal and bowel bacteria.
The impact of infectious disease
The description of infectious disease is often the subject of one of the first chapters in general textbooks about pathology and clinical medicine. This is a reflection of its clinical significance. In terms of causation of disability and death, infections are the single most important cause of disease worldwide. However, in developed countries, infections do not have quite the overwhelming impact as they do in developing countries. In the developed world the most important causes of death and disability are now the non-communicable diseases, and most importantly, disease of the cardiovascular system, cancer and, in young people, accidents.
The modern developments of vaccination and drug treatment for infections are widely acknowledged in conventional medicine to have led to the decline in infectious disease in developed populations. However, it is increasingly understood that the provision of basic needs, such as clean water and the removal of sewage (sanitation), good housing and adequate diet, are also important factors that account for the reduction in infection-related morbidity. The provision of these basic needs contributes to the attainment of a basic level of health, and in health, the body is better able both to withstand contracting some infections, and also to overcome those infections that do develop.
The study of the epidemiology of diseases such as tuberculosis (TB), rheumatic fever and diphtheria supports this theory that healthy populations resist infectious disease. All these diseases have declined dramatically in well-nourished Western populations, and yet are major causes of ill health in developing countries. For all three, the decline preceded the introduction of vaccinations or antibiotic treatment, and this is understood to relate to improvements in nourishment and sanitation. The reports that TB is on the increase again in Western countries reflect an increase largely confined to marginalized groups, such as those people who are homeless, who do not have access to good nutrition and housing, to those who have recently emigrated from developing countries, or those with HIV (human immunodeficiency virus) infection/AIDS (acquired immune deficiency syndrome).
Measles is an infectious disease from which most well-nourished children make a full recovery, with less than 0.1 percent developing long-term serious consequences. However, measles is still one of the major causes of infant death and blindness in developing countries, causing death in up to 30 percent of infants who contract it in some impoverished localities. This modern statistic illustrates the fact that infectious disease still has much more impact in populations that are more susceptible as a result of malnutrition and poor living conditions.
Nevertheless, there are many infections that cause serious disease within healthy populations. Examples of such diseases include bacterial meningitis, certain types of pneumonia, certain types of food poisoning (including Escherichia (E.) coli serotype O157), HIV infection and many of the tropical diseases (e.g. malaria and typhoid). Although it is true that these diseases often tend to have a greater impact on those who are more susceptible, such as the very young and elderly people, they are also well recognized to be the cause of significant ill health and mortality in previously fit young people. This is an indication that some infections can take hold and cause damage even when the individual does not obviously appear to be particularly susceptible to a severe illness.
Immunity and infectious disease
The topic of immunity was introduced in Section 2.1d. The general level of immunity to a disease in a population is another major factor that affects the impact of the disease. Diseases that are very common in a population are termed endemic. Chickenpox and the common cold are diseases that are endemic in the UK, whereas malaria is endemic in a tropical country such as Uganda. It is common for a population in a locality that has been exposed from birth to an endemic disease to have less marked reactions to that disease than visitors to that locality. This is why travelers to foreign countries frequently succumb to infections that do not seem to affect the locals, leading to symptoms such as fevers, diarrhea and vomiting.
The resistance of a population to an endemic disease is attributed in part to herd immunity. This term describes the immunity that results from the fact that a large proportion of the population has already encountered that infection, often in infancy. For this reason, they will not thereafter easily contract the disease. If herd immunity exists, the disease is less likely to take hold in the population because there are fewer people to whom it can be passed on. This is one of the principles of vaccination campaigns that aim to ensure that a large proportion of the population is immune. The end result of a successful vaccination campaign is an increase in the herd immunity to a specific disease such as measles.
However, acquired herd immunity is not the whole story. Local people who have not yet experienced endemic diseases still generally appear to be able to fight off these infections when they do experience them more efficiently than travelers to the area. A striking example of this is the impact that was made by minor infections taken by the first explorers to the New World. Diseases such as the common cold and chickenpox caused serious consequences in communities to whom they were totally new infections. It seems as if the immune system of a person in a locality is prepared in some way to deal with an endemic disease, but is seriously challenged by one that is not a normal part of the local pattern of infections. This extra level of protection may possibly be the result of a genetic adaptation of the local population over generations of exposure to endemic diseases.
If a disease suddenly appears to be on the increase in a population, the term epidemic is used. An epidemic disease is not necessarily severe, but occurs much more often than expected. Epidemics occur either because a new type of infectious agent has emerged for which there is no pre-existing herd immunity, or because over the course of time the herd immunity has diminished so that the disease can rapidly take hold once again.
The pathology of infectious disease
There are two properties of infectious agents that result in them being able to cause harm. The first is that they have the ability to penetrate the normal barriers to disease, and the second is that, having penetrated, they are able to damage the body tissues.
An example of these two stages is the damage caused as a result of flea infestation. The flea can penetrate the barrier of the skin because it is able to puncture it by biting. Damage is due to substances in the flea’s saliva that enter the deep layers (dermis) of the skin. These substances are irritating to the cells in the dermis, and so cause inflammation at the site of the puncture. This leads to the small red and itchy bump of the flea bite. In some people these chemicals behave as antigens and provoke a pronounced immune response. This leads to the site of the bite becoming firmer and larger and even blistered over the course of a few days.
Infectious diseases can lead to damage as a result of one or more of the seven processes of disease described in Section 2.2a. The flea bite is an example of how an infectious agent can give rise to inflammation, which is the most common disease process to result from infectious disease. However, infectious agents can also lead to disease by causing problems in the immune system, causing cancerous change, and also by causing degeneration.
The following descriptions of infectious diseases illustrate the diversity of the disease processes involved.
TONSILLITIS
The tonsils are masses of lymphoid tissue (like lymph nodes) that sit at the entrance to the pharynx (they can be seen in most people at the back of the mouth, just behind the arch formed by the palate). Tonsillitis can be passed on from one person to another by the inhalation of droplets of infected fluid, following a cough, for example. However, tonsillitis is not always contagious. It can also occur when a person becomes susceptible to relatively harmless bacteria that they have been harboring in their tonsils for some time. This might explain how some people get recurrent bouts of tonsillitis whenever they are run down.
The symptoms of tonsillitis are primarily due to inflammation of the tonsils. The bacteria and viruses that cause tonsillitis are able to attach to the epithelial cells of the surface of the tonsil by means of chemicals (proteins) on their coating. Damage is caused partly because the microbes can kill tonsil cells by first attaching to them, and as a result they stimulate inflammation. Some bacteria encourage the excessive production of pus, consisting of dead phagocytic leukocytes. The chemicals released during the immune response and inflammation give rise to the general feelings of malaise that are common in tonsillitis. In most cases the immune response is sufficient to hinder the spread of infection so that it settles down within a few days.
COLD SORES (HERPES LABIALIS)
The herpes simplex virus is transmitted through intimate contact such as kissing. The virus is present in the saliva of the carrier, and from there penetrates into the cells of the lining of the partner’s mouth, where it initially, in a quasi-parasitic way, uses the supply of nutrients within the mouth’s lining cells to replicate itself. It also penetrates the ends of nerve cells in the connective tissue of the lining of the mouth, and this leads to the characteristic tingling sensation. Eventually, the infected cells will rupture and die, and cause inflammation, which is the cause of the redness and scabbing. Each ruptured cell will release thousands of new viruses into the saliva of the person with the cold sore.
Intense pain can result as an effect of the infection of the nerve cells. When the infection has healed, some viruses remain latent within some of the nerve cells, but these are prevented from replicating by the immune system. When the person becomes run down, this hidden source of infection can emerge as another cold sore.
ACQUIRED IMMUNE DEFICIENCY SYNDROME (AIDS)
HIV is carried in body fluids. Close contact with infected body fluids together with a break in the normal barrier of the skin is necessary for HIV to be transmitted. Sexual contact and childbirth provide these conditions, as does transmission of human fluids by a hypodermic needle.
The presence of the virus in the bloodstream leads to an immune response that then causes the flu-like syndrome characteristic of many viral infections. Although antibodies are formed, they are most often unable to clear the HIV from the body. This is because the virus is able to penetrate into leukocytes via the CD4 protein receptor, and in this way hide from the immune response. In this intracellular state the HIV remains and multiplies, dividing with the leukocytes over a period of years, until gradually the function of those cells becomes impaired. The presence of the virus leads to gradually reduced numbers of lymphocytes over time as their early death is promoted by the presence of the virus. Therefore, the major consequences of HIV infection are not primarily due to inflammation, but instead to a disorder of the immune system. This disorder leads to the combined problems of opportunistic infections and cancer.
In advanced disease, the nerve cells of the brain may also be damaged by cytokines produced by virus-infected immune cells, leading to a form of dementia. AIDS is, therefore, an example of an infection causing degeneration as well as inflammation and a disorder of the immune system.
These diverse examples illustrate how infections cause disease by various mechanisms. Although inflammation is the most common damaging consequence of infections, autoimmune disease, immunodeficiency, cancer and degeneration can also be caused by them. Infections can also cause congenital disease in the embryo and fetus if contracted during pregnancy, such as congenital rubella syndrome, which results from a rubella (German measles) infection during the first trimester of pregnancy.
Transmission of disease
The three examples of infectious diseases described above also illustrate the diversity of ways in which infections can be transmitted. Tonsillitis is transmitted by inhalation of droplets, the cold sore virus through infected saliva penetrating the mucous membranes of the mouth, and HIV is transmitted via infected body fluids penetrating the usual skin barriers.
There are an enormous number of possible routes of infection, illustrating the complex ways by which infectious agents have adapted themselves to find their way into their human hosts. Modes of transmission can be broadly considered in two categories: person-to-person and animal-to-person spread. Examples of these are listed in Table 2.4a-II.
Table 2.4a-II Different modes of transmission of infectious agents |
Person-to-person spread |
Infection in the gastrointestinal tract passed on through contact with human feces (usually indirectly by eating contaminated food or water), e.g. typhoid and cholera |
Infestations passed on through clothing and bedding, e.g. body lice |
Animal-to-person spread |
Infection carried by insects and mites passed on through bites, e.g. malaria |
Infection passed on through contact with animal feces, e.g. food poisoning due to bacteria such as Salmonella and E. coli originating from poorly cooked contaminated meat and eggs |
Infection passed on through eating meat containing parasites, e.g. tapeworm |
Infection carried in rat urine that penetrates the skin via contaminated water, e.g. Weil’s disease |
For each of the known infections, the causative infectious agent has adapted to target particular human tissues and causes damage in a characteristic way. The conventional view is that such adaptation is the consequence of evolution over the course of many years. Evolution is believed to be the result of mutation that is beneficial to the infectious agent. As a species of microbe replicates, mutation occasionally leads to daughter microbes that are better able to reproduce than their parents. In the case of infectious disease, this successful adaptation is often one that leads the daughter microbes to be more efficient at infecting human cells. This is because it is through infection that microbes obtain the nutrients necessary for survival and replication.
Evolution in complex species such as humans is believed to occur very gradually over many thousands of years. However, evolution of simple species such as viruses and other microbes may lead to a dramatic change in a species over a much shorter time. This is because the microbes replicate at a very rapid rate.
HIV is an example of an infection that seemed to emerge, probably originally in Africa, within the last 50 years, before which time it was completely unknown. HIV currently appears to be perfectly adapted to thrive in the human body through having a protein on its coat that connects exactly with the CD4 proteins found on various leukocytes of the immune system. One theory is that HIV previously existed as a monkey virus,48 and through chance mutation acquired this protein that enables it to survive and replicate in humans.
New microbes are emerging in this way all the time, as they change in form through replication. Over the past quarter of a century, more than 30 new or newly recognized infections have been identified around the world. The newly emerging infectious diseases include hepatitis C and E viruses, Lyme disease, staphylococcal toxic shock syndrome, H5N1 pandemic influenza, SARS (Severe Acute Respiratory Syndrome) virus and nvCJD (new variant Creutzfeldt-Jakob disease) virus.
Mutation not only gives rise to new species of infectious organism; it can also lead to subtle changes in existing organisms. These changes, which lead to new strains, can lead to organisms no longer being recognized by the immune system of someone who previously had immunity to that disease.
This type of mutation appears to occur more readily in some infectious organisms than others. For example, the influenza virus rapidly changes its characteristic coat through mutation. This gives rise to the frequent epidemics of influenza that are experienced in the UK. Each influenza epidemic reflects a wave of infections by a new strain of the virus. This explains why people can contract influenza repeatedly during their life, and why a new vaccine for influenza needs to be developed on an annual basis.
In contrast, the chickenpox virus appears not to mutate significantly, reflected in the fact that most people retain their immunity to chickenpox for life.
2.4b The different types of infectious disease
Infectious organisms
The main groups of infectious agents were introduced in the last section and summarized in Table 2.4a-I. Each one of these groups is now considered in turn, together with some of the diseases that these diverse agents can cause.
Diseases caused by viruses
Unlike bacteria and human cells, viruses and prions are so small that they cannot be seen using a simple microscope. Instead, the more powerful electron microscope is used to produce images of these particles. Figures 2.4b-I and 2.4b-II show electron micrographs of two different viruses: the envelope-bounded DNA of the herpes simplex virus, and the icosohedral (20-sided) RNA of the rotavirus. The size of these viruses is measured in nanometers (nm), which is an unimaginable one-billionth of a meter (1 × 10-9 m) in size. These viruses are 160 and 80nm in diameter, respectively. By way of comparison, a human red blood cell is 6000–8000nm in diameter, so up to 100 times larger than these tiny infectious particles.
Figure 2.4b-I Electron micrograph of the herpes simplex virus
Figure 2.4b-II Electron micrograph of the human rotavirus
Viruses contain some genetic material that is similar in structure to that found within human chromosomes, together with additional proteins that form a surrounding coat. Prions are composed of a single complex protein alone. Because of their simplicity, viruses and prions often have a structure that resembles a geometric crystal. This is in contrast to the complex and unique rounded structures found in each living cell.
Viruses cause damage because they have adapted to be able to penetrate living cells, and then to utilize the nutrients within those cells to replicate. Viruses cannot replicate, obtain nutrients or respond to environmental changes without living cells and so cannot, by definition, be considered a life form.
Viruses cause disease both because they damage living cells, and also because they trigger an immune response. The high temperature, rash, aches and malaise characteristic of many viral infections are manifestations of the immune response generated against viruses circulating in the bloodstream.
Viruses can damage human cells in different ways: some simply cause cell death, as, for example, does the cold sore virus. This can have many consequences, depending on which tissue is affected. The death of human cells and release of cellular contents leads to inflammation, so the familiar features of redness, heat, swelling and pain are commonly found when part of the body is affected by a virus.
Other consequences include:
•disturbance of cell replication, leading to benign growths (e.g. wart virus)
•cancer, as a result of interfering with the proto-oncogene function in a cell (e.g. genital wart virus, which can be a trigger for cervical cancer)
•disturbance of the development of the fetus, leading to congenital malformation (e.g. rubella virus) if the infection occurs in pregnancy.
An excessive immune response to viruses can also be damaging. Possible consequences include:
•bleeding within tissues (e.g. the viral hemorrhagic disease Lassa fever)
•delayed immune damage to the nervous system, which can manifest as numbness, weakness and mood changes (e.g. Guillain-Barré syndrome).
The poorly understood post viral fatigue syndrome (PVFS), which describes depression and exhaustion following a viral infection, has also been attributed to delayed immune damage to the nervous system following a viral infection.
Viruses are classified into groups (families and subfamilies) according to their structure. Some of the major families of viruses are listed in Table 2.4b-I, and some of the diverse diseases that result from viral infections are listed in Table 2.4b-II.
Table 2.4b-I Some different families of virus | |
Class of virus | Description |
Adenoviruses | A large group of DNA-containing viruses that commonly cause minor upper respiratory infections |
Herpesviruses | DNA viruses responsible for cold sores, chickenpox and glandular fever. All are capable of causing latent infections that can reappear at a later date |
Poxviruses | DNA viruses causing crusting diseases such as smallpox, cowpox, monkey pox and orf |
Picornaviruses | Small RNA viruses causing diseases including poliomyelitis, viral meningitis and the common cold |
Reoviruses | Small RNA viruses responsible for epidemics of infectious diarrhea and mild respiratory symptoms |
Retroviruses | Viruses that are able to replicate by inserting a protein called reverse transcriptase into a cell. Includes HIV |
Disease | Causative virus |
Pharyngitis (sore throat) | Adenovirus |
Cold sores | Herpes simplex |
Genital herpes | Herpes simplex |
Chickenpox | Varicella zoster |
Viral encephalitis | Measles, mumps, herpes simplex |
Viral meningitis | Coxsackie virus, measles, mumps |
Rabies | Rabies |
Croup | Respiratory syncytial virus |
Glandular fever | Epstein-Barr virus |
Kaposi’s sarcoma | Epstein-Barr virus |
Smallpox | Variola |
Poliomyelitis | Poliovirus |
Mumps | Mumps virus |
Lassa fever | Lassa virus |
Viral pneumonia | Measles, varicella zoster |
Diarrhea | Rotavirus, norovirus, adenovirus |
Rubella (German measles) | Rubella |
Yellow fever | Yellow fever |
Influenza | Influenza |
Measles | Measles |
Common cold | Rhinovirus |
Diseases caused by prions
Prions have only recently been discovered and described as infectious agents. They have a very simple structure consisting of a single folded protein, making them the only infectious agent that doesn’t contain the self-replicating material DNA or RNA. Prions are believed to cause disease by inducing prion-like folds in the structure of essential proteins in the nervous system of their animal hosts. This leads to an irreversible change into useless complex protein structures. This leads the host cells to degenerate, and enables the prion to increase in quantity. Creutzfeldt-Jakob disease (CJD) and the new variant form (nvCJD) are two prion diseases that have been shown to affect humans. As far as it is known, prion diseases can only be transmitted when infected animal tissue or blood enters the body of another animal, either by the ingestion of infected flesh, or through surgery and possibly through blood transfusion.
Diseases caused by bacteria
Bacteria (singular bacterium) are microorganisms that consist of a single simple cell. The structure is far less complex than the typical animal cell described in Section 1.1b. Bacteria do not have nuclei or mitochondria, for example, but instead, the copying of genetic material and the energy-generating processes all take place around large molecules that are situated free within the cell cytoplasm. Nevertheless, the bacterium still uses basic nutrients and reproduces in a similar way, and for this reason, a bacterium is considered a life form. Bacteria are, on average, about one-tenth of the size of human cells, but can be seen through a simple microscope.
Figure 2.4b-III shows a simplified drawing of a rod-shaped bacterium, Escherichia (E.) coli. The size of bacteria is generally measured in micrometers (μm), which are one-millionth of a meter (1 × 10-6m) in size. An E. coli bacterium is 3μm in length (3000nm), which is 20 times wider than the diameter of the complex herpes simplex virus, and half the diameter of a red blood cell.
Figure 2.4b-III Simplified diagram of a coliform bacterium (Escherichia coli)
If a bacterial disease is suspected, a doctor might take a swab of the infected site, for example, an inflamed tonsil. The swab is taken to the laboratory, where it is swept across a glass slide, stained with a dye that is absorbed by bacteria, and the smear then viewed through a microscope. In the examination of a smear from an inflamed tonsil resulting, for example, from infection by the Streptococcus bacterium, the microscope might reveal clumps of large epithelial cells from the tonsil, and chains of tiny darkly stained circular forms, the streptococcal bacteria (also known as streptococci).
More recently, bacteria have been classified according to their genetic makeup. This has led to the reclassification of some groups, because genetic analysis reveals similarities and differences that are not apparent from their morphological characteristics.
Important Gram-positive bacteria include species of Staphylococcus, Streptococcus, Bacillus, Clostridium (including tetanus), Mycobacterium (including tuberculosis), Corynebacterium (including diphtheria) and Listeria.
Important Gram-negative bacteria include species of Chlamydia, Escherichia (e.g. E. coli), Salmonella, Vibrio (e.g. cholera), Helicobacter, Neisseria (e.g. gonorrhea and meningococcus), Bordetella (e.g. whooping cough), Legionella (e.g. Legionnaires’ disease) and Treponema (e.g. syphilis).
Some infectious diseases, for example, tonsillitis and pharyngitis, can be caused by both bacteria and viruses. Other examples include meningitis, encephalitis, pericarditis, bronchitis and pneumonia. Although in these conditions there are a range of possible infectious agents, the end result of the infection is similar in terms of the development of inflammation within a particular organ.
Many infectious diseases are described by terms that end with the suffix “-itis.” However, this suffix refers only to the fact that an organ is inflamed, but not what the cause might be. This is also true for the term pneumonia that describes inflammation in the lung tissue and not the causative agent. In fact, some inflammatory conditions, including bronchitis, pneumonia, cystitis, pericarditis and meningitis, can also be the result of damage by chemicals, radiation and autoimmune disease. They are, therefore, not always infectious diseases.
Diseases caused by protozoa, flukes and worms
Protozoa are single-celled animals that share all the characteristics of the typical animal cell. They multiply by simple binary fission (mitosis).
Flukes, tapeworms and worms are more complex multicellular animals that possess a rudimentary nervous system and gut. They multiply by producing eggs that develop through an immature stage to the adult form. Often an additional animal host to the human is required for the life cycle to complete itself. For instance, the eggs of the pork tapeworm have to mature in the flesh of pigs before the parasite can be passed on to humans (as a result of eating infected meat).
Collectively, protozoa, flukes, tapeworms and worms are considered to be parasites when they infect human beings. The scientific definition of a parasite is a plant or an animal that lives in or on another living entity and at the expense of that entity (plant or animal). In medicine this term is not used to describe bacteria or viruses. Nevertheless, the damage wreaked by parasites is similar to that resulting from infection with bacteria and viruses, and includes inflammation and an excessive immune response.
Protozoal diseases include giardiasis and amoebiasis (both causes of diarrhea), malaria and sleeping sickness (tropical disease), and Trichomonas infection (a sexually transmitted disease). Most worm and fluke infections are tropical diseases, although threadworms and toxocariasis are worm infections that are commonly contracted in the UK.
Diseases caused by fungi
Fungi are a large class of life forms that includes the familiar mushrooms and toadstools, but also molds and yeasts. Strictly speaking, they are classified as part of the plant kingdom.
Fungi are different from all other plants in that they do not contain the green pigment called chlorophyll. Because of this they cannot use sunlight to manufacture energy. Instead, fungi require living or rotting animal and plant tissue to survive. Therefore, many have evolved to be parasites, obtaining nutrients from other living organisms, including humans.
In general, fungi do not cause serious disease in humans unless there is depletion of the immune system. In people with immunodeficiency, fungal disease can become a significant and sometimes life-threatening problem. However, minor fungal skin and mucous-membrane infections are very common in healthy people. These include athlete’s foot, ringworm and thrush.
Athlete’s foot is generally a mild fungal infection that targets damp skin. The most common site for the infection to start is the skin between the fourth and fifth toes. It rarely spreads beyond this area, although it can affect the growing nails, resulting in thickening and discoloration.
Ringworm is a fungal skin infection that forms an itchy, spreading ring of redness and scaling on the skin in a way that can be compared to the “fairy ring” produced by some mushrooms. This mild but very contagious rash is often contracted from handling animals.
Thrush is due to a yeast called Candida albicans, which is a natural resident (commensal) of the bowel and genital tract of healthy human beings. It becomes problematic when the natural balance of the microorganisms in the body becomes disturbed. The candida then becomes too dominant, akin to overgrowth of one plant species in a poorly tended garden. Excessive candida leads to thick white patches and discharge, and can cause inflammation of the mucous membranes of the genital tract and the mouth.
Candida syndrome, first widely publicized by William Crook in 1984,49 is recognized by some complementary health practitioners as a complex constellation of symptoms that include digestive problems, such as diarrhea, irritable bowel syndrome and bloating, other fungal problems, such as athlete’s foot, and mental problems, such as exhaustion and depression. Thrush may or may not be present in candida syndrome. The symptoms of chronic fatigue syndrome (CFS) (also known as myalgic encephalomyelitis or ME) have also been attributed to candida. Excessive sugars and yeasts in the diet are claimed to be one of the causes of the candida syndrome, and so the treatment involves a strict diet and the use of antifungal preparations. The use of preparations of the natural bacterium Lactobacillus acidophilus is recommended to help regain a healthy natural balance of microorganisms as well as long-term antifungal medication. However, candida syndrome is not currently recognized as a disease entity in conventional medicine.50 For this reason, patients who believe that they have candida syndrome will not routinely be offered antifungal treatment by conventional doctors unless they have the white discharge, soreness and itching characteristic of genital or oral thrush. The clinical evidence supporting the benefits of Lactobacillus supplementation or long-term antifungal medication in chronic fatigue and minor digestive disturbance is also not believed to be strong. Nevertheless, many doctors will recommend Lactobacillus supplementation to treat depletion of these bacteria resulting from antibiotic treatment or for the treatment of thrush.
Fungal infections are very problematic for someone with immunodeficiency. For example, patients with AIDS commonly experience severe thrush outbreaks, which can affect the whole esophagus and mouth as well as the genital mucous membranes. In severe immunodeficiency, candida can spread to invade the lining of the bowel and other deep parts of the body, and this can be life-threatening. If HIV progresses to AIDS, then patients may also develop a skin condition called seborrheic dermatitis, which is also due to overgrowth of a usually harmless skin fungus, Malassezia. Unusual fungal infections, such as cryptococcosis and pneumocystis pneumonia, are common causes of death in patients with immunodeficiency syndromes such as AIDS. Both these species of fungi are found as commensals in the microbiome of the healthy person (see below).
Infestations
Infestations by arthropods such as lice, scabies and fleas are different in nature to the other infectious diseases, as it is generally only the surface of the body that is affected. Biting and burrowing by these creatures causes damage to skin tissue and allergic responses, which then give rise to itching and discomfort.
Although the itch and discomfort can cause moderate distress, infestations are a cause of serious disease because arthropods can carry other diseases that are transmitted through bites. The Black Death, which caused millions of deaths in Europe in the 14th century CE, is a disease transmitted through the bite of rodent fleas and caused by the Gram-negative cocco-bacillus Yersinia pestis. Other arthropod-borne infections include typhus (tick-borne) and malaria (mosquito-borne). Many other tropical diseases are also transmitted via insect bites.
Beneficial (commensal) microorganisms
In the healthy human being, as in all other large animals, there are thousands of bacteria and fungi resident throughout the body, totaling at least the number of human cells that make up that body.51 Microorganisms can be found on all epithelia that are in contact with the outside world, including the skin and the epithelial linings of the lung, bowel and genital organs. A person starts to inherit these guests from the moment they are born and come into close contact with caregivers. Newborn infants are very prone to episodes of oral thrush and colic, possibly an indication that the delicate balance of fungi and bacteria has yet to become established. It is estimated that a balanced microbiome is not established until one year of life. This population of microorganisms is dynamic and the genetic balance highly responsive to environmental changes. One study showed that exposure to breast milk, fever, baby rice, antibiotics and final adult foods all induced increases in complexity of the microbiome in babies in their first year.
Each human being can be considered as a distinct ecosystem, in which ideally the human host and its microorganisms live in harmony. It is known that the health of the human depends in part on the balance of these microorganisms, also termed the human microbiome. For example, some bacteria in the bowel assist with the digestion of food. It is noteworthy that the medical term for these healthy bacteria is commensal, literally meaning that they share the same dining table as their hosts. Also, a healthy balance of microorganisms is known to prevent the development of infection by less desirable infectious agents. The use of antibiotics and excessive washing (e.g. of the genital areas) can alter this balance, and this is why these practices may lead to problems such as diarrhea, thrush and soreness. Imbalances in the microbiome have been linked with diseases as diverse as obesity, psoriasis, bacterial vaginosis and esophageal cancer, and this is the focus of a burgeoning area of scientific research.
As a result of this new awareness of the microbiome, there is a rationale for supplementing the friendly commensal bacteria of the body with dietary products containing bacteria such as the Lactobacillus species (sp.) (the family of Gram-positive, rod-shaped bacteria, which include those responsible for the conversion of milk to yogurt) and other fermented products such as miso, sauerkraut and tempeh. It is recognized that oral supplementation with preparations of live Lactobacillus sp. will not only encourage the colonization of the bowel with these bacteria, but also the external genitalia and the vagina. Lactobacillus sp. are recognized to increase the acidity of their environment, and in this way to reduce the overgrowth of yeast-like organisms such as candida, the cause of thrush.
The precise balance of the biome is unique to individuals, but similarities are shared between cultural groups. Close contact between individuals will lead to shifts in the balance of microorganisms in the biome, and this is bound to happen to a certain extent between groups of small children and their families.52
The long-term consequences of powerful medical treatments including antibiotics and vaccinations on the biome are not by any means fully understood. One study showed that a broad-spectrum antibiotic left the microbiome in the gut markedly denuded of bacterial species, but some individuals recovered their bacterial diversity much more rapidly than others. Some individuals still demonstrated the effects for up to four years following the antibiotic treatment. Clostridium (C.) difficile is a bacterium that can overgrow and cause life-threatening chronic diarrhea in vulnerable patients who have been given broad-spectrum antibiotics, presumably because competing bacteria have been eliminated. Ironically the main treatment for this is another very powerful broad-spectrum antibiotic. Novel approaches to this problem are being explored with trials of fecal transplants containing bacteria from donors. These have shown benefits in intractable C. difficile infections.53
Studying infectious diseases
Knowledge of the classification of infectious agents and their modes of transmission, together with an understanding of how infectious agents can lead to processes of disease, is invaluable in the study of infectious diseases. Moreover, it can help in understanding how the different broad groups of infectious agent can each lead to some characteristic manifestations of disease. Table 2.4b-III illustrates some features of diseases that can point to a particular type of infectious agent, but there are many exceptions to these rules.
Table 2.4b-III Characteristics of disease caused by infectious agents | |
Infectious agent | Features of disease that are characteristic of the agent |
Viruses | Can cause a syndrome of malaise, joint aches, mild fever and slight rash as infection takes hold |
Lead to watery discharges more commonly than pus | |
Rarely, can lead to cancerous change or benign tumors | |
Infections may be followed by a syndrome of prolonged exhaustion, depression, muscle weakness and numbness | |
Prions | Transmitted by insertion of infected tissue into the body either by eating or by medical procedures |
Lead to degeneration of nervous tissue | |
Bacteria | Pus formation, such as abscesses and boils, may occur |
The most common disease process in bacterial disease is inflammation. May also trigger autoimmune responses | |
Protozoa, flukes and worms | A common cause of tropical intestinal disease. Eggs of parasites often enter the bowel from contaminated food and water |
Allergic reactions, such as itch and cough, may also occur | |
Fungi | A cause of relatively mild skin problems in healthy people |
A major problem in patients with immune deficiency | |
Infestations | A cause of irritating skin problems, but do not in themselves affect deeper tissues |
May lead to serious disease by transmission of other infectious agents | |
•causative agent
•mode of transmission
•clinical features
•complications of infection
•treatment.
For example, a textbook description of the infectious disease measles might give the following information:
•Causative agent: a virus from the family of paramyxoviruses.
•Mode of transmission: the spread is by droplets (infected fluid particles from the nose and throat of a carrier). A person is infectious for a total of six days. The infectious period begins four days before the rash develops to two days after it has appeared.
•Clinical features: these occur after an incubation period of 8–14 days. This means that the disease will only manifest 8–14 days after contact with a carrier. First, malaise, fever, runny nose (rhinorrhea), cough and redness of eyes (conjunctival suffusion) occur. This is when the patient is infectious. After two days of this, gray spots (known as Koplik spots) develop in the inside of the mouth. These are diagnostic of measles infection. On day four, a rapidly spreading maculopapular (spotty, slightly raised) rash moves from the face to include the whole body.
•Complications: measles is generally a mild infection in well-nourished people. However, complications are common in poorly nourished people, and include pneumonia, bronchitis, ear infection and bowel infections. A serious long-term brain infection is a very rare complication in infections in infants less than 18 months old. Measles, when caught in pregnancy, can lead to loss of the pregnancy, but not to fetal abnormalities.
•Treatment: there is no direct treatment of the infection available. Measles in a healthy person generally requires only supportive treatment. However, if the patient is vulnerable because of immunodeficiency or malnutrition, or if neurological or respiratory signs develop, hospital admission and intravenous nutrition may be necessary.
Information about infectious diseases structured under these five headings encapsulates the essential facts required for medical practitioners to understand how a disease will impact on the life of a patient. Awareness of these headings can help structure information for the purposes of the study of infectious diseases.
Information box 2.4b-I
Infectious disease: comments from a Chinese medicine perspective
The concept of epidemic disease was well established in China from at least the 3rd century CE. It has been understood for centuries that certain diseases with contagious properties and manifesting with Heat symptoms could progress through deepening levels of the body with more serious consequences. These diseases no doubt included viral upper respiratory illnesses, and also more serious life-threatening diseases such as epidemic influenza, smallpox, plague, typhoid and typhus. These diseases were seen to be more likely to seriously afflict those who had Deficiency. For this reason, prevention by Nourishing Deficiency was fundamental to the management of epidemics. Whilst infectious agents were not recognized, epidemic diseases were understood in terms of Invasion of Wind Heat or Heat Toxins. Theories such as the Four Levels (Wen Bing) and Six Divisions (Shang Hun Lun) explained these diseases. These were the foundation for herbal formulas, the ingredients of some of which in recent times have been found to have antibacterial and antiviral properties. The classic treatment principles were to Clear Wind Heat, Clear Toxins, Resolve Heat, and very importantly, to Tonify and Nourish Qi and Blood.54
However, not all diseases that are described as infectious in conventional medicine will equate to these classically recognized epidemic warm diseases.
To help us to describe conventionally described infectious diseases in Chinese medical terms, two important concepts need to be grasped. These are:
•The origin of the Pathogenic Factor. Chinese medicine describes how the Pathogenic Factor that is manifested in a conventionally recognized infectious disease (e.g. Wind Heat) can originate from outside the body (i.e. externally, e.g. climatic Wind Heat) or from within the body (i.e. internally, e.g. Heat from Stagnation of Qi and Blood), or can be a combination of the two.
•The depth of the Pathogenic Factor. According to Chinese medicine, the Pathogenic Factor may be situated either on the Exterior of the body or at a deeper level in the Interior. Maciocia (1989)55 makes the point that, in practice, the origin of the Pathogenic Factor is far less relevant than the depth at which it is located in the body. However, the depth of the Pathogenic Factor does not tell us about its origin. The depth is an important aspect of the identification of patterns according to the Eight Principles, which are described in accessible language by Maciocia (1989).56
It is important to be very clear about the distinction between the origin of a Pathogenic Factor (External/Internal) and its location/depth (Exterior/Interior). It is also important not to conclude, because of the poetic nature of Chinese descriptions such as “Wind Heat has invaded the body and caused disease,” that the Pathogenic Factor is synonymous with an infectious agent. This is not the case. The Pathogenic Factor is actually a description of the clinical picture (e.g. redness, fever and pain indicate Heat; rapidly changing symptoms indicate Wind).
The reverse is also true; microbes are not synonymous with the Chinese medicine Pathogenic Factors. However, according to an integrated view of infection, microbes can and do give rise to manifestations (Invasion) that are taken to be evidence of Pathogenic Factors. This means that the typical signs and symptoms of infectious diseases from a conventional viewpoint have some close correlations with the Chinese medicine view of Pathogenic Factors, and can inform a Chinese medicine diagnosis.
The origin of the Pathogenic Factor
In the case of an infectious disease interpreted in Chinese medicine terms, the actual origin (cause) of a Pathogenic Factor may be Internal or External, depending on the situation. For example:
•Pathogenic Factor of External origin. When the Pathogenic Factor has an External origin, its manifestation can be put down purely to the energetic effects of a factor that is manifest in the external environment. This could be an infectious agent, but could also be an allergen, a carcinogen or other environmental factors such as climate and diet. For example, a virus infection and exposure to a cold wind may together lead to the manifestation of Wind Cold on the Exterior in an otherwise healthy person.
Another example is a case of vomiting and diarrhea in a healthy person after eating contaminated food. The Chinese medicine interpretation might be Invasion of Damp Heat in the Intestines. This describes the manifestation of the condition. In this case, the origin of the Pathogenic Factor is External, and the development of the syndrome has been triggered by the toxins produced by the bacteria in the contaminated food.
Finally, one more example of an infectious disease leading to the appearance of a Pathogenic Factor in this way might be any one of the contagious infestations such as scabies and head lice, which can affect very healthy people who come into contact with them. The itch and rash associated with these diseases are symptoms of Heat, but follow the damage to the skin by mites. In both these infestations, the mites are very irritating to the skin, and based on the signs and symptoms it would be appropriate to describe their bites and burrowing as having led to a Channel Invasion of a Pathogenic Factor (in this case, Wind Damp Heat).
•Pathogenic Factor of Internal origin. When the Pathogenic Factor has an Internal origin, the manifestation of an illness is an appearance of a deeper imbalance (such as pre-existing Damp Heat). If the illness is considered to be an infectious disease in conventional medical terms, this infection is an expression of a deep imbalance, rather than a new condition. An example of this might be the appearance of cystitis (Damp Heat in the Bladder) in someone who has pre-existing Damp Heat and Kidney Deficiency. In this example, the infectious agent according to conventional medicine terms is usually a microbe that normally forms part of the human ecosystem and which has multiplied out of its usual state of balance to lead to the symptoms of inflammation. If the person regained their healthy state of balance the manifestations would disappear.
The crucial difference between these two categories is that, in the former, an infectious agent (akin to a strong Pathogen) has contributed to the generation of an imbalance, and in the latter the infection appears as a manifestation of an underlying deep imbalance for which the infectious agent is an aspect of its expression. In all probability, the symptoms of many infections are a result of a combination of the two, in that the energetic property of the infectious agent triggers the manifestation of a Pathogenic Factor in a person who already has some degree of underlying imbalance.
The distinction between these three categories is of fundamental importance in using the classifications of conventional medicine to inform a Chinese medicine diagnosis.
The depth of the Pathogenic Factor
Over the centuries, the Chinese medicine interpretation of those diseases that involve fever evolved into theories that acknowledge that a Pathogenic Factor can manifest at various levels in the energetic body. Three of these theories identify patterns of disease according to “The Six Stages,” “The Four Levels” and “The Three Burners,” respectively.57 According to these interpretations, it is possible to be more precise about how deeply the Pathogenic Factor is manifested in the Interior.
These patterns of differentiation often reflect how diseases progress in a conventional medicine perspective.
The energetics of two common infectious diseases
When interpreting the energetics of infectious disease, the important questions that need to be addressed are:
•What Pathogenic Factors are manifesting (consider symptoms and signs)?
•Are the Pathogenic Factors manifesting on the Interior (organs) or Exterior (skin and muscles – the domain of the Jing Luo)?
•What is the origin of the Pathogenic Factor (has it come from without, or has it been generated internally)?
There are several important guidelines to bear in mind. These are:
•Infection will always represent the presence of a Pathogenic Factor.
•The person who develops the disease may already have a degree of Depletion of Blood, Qi, Yin or Yang. An already present Pathogenic Factor such as Damp or Heat may also be complicating this deficiency. This will affect how the symptoms of disease manifest.
•Characteristics that suggest that the manifestation of the Pathogenic Factor results from an energetic property of the infectious agent (External origin) include a high level of infectiousness and symptoms that are strongly characteristic of a particular type of infectious agent.
•Infections that result from exposure to extremes of climate, or other miscellaneous causes of disease (e.g. unclean food), also suggest a Pathogenic Factor of External origin.
•Infections that are not easily contagious but which recur in the same person over and over again suggest a Pathogenic Factor of Internal origin.
•Wind is often present in infectious disease, and is suggested by any symptoms that change rapidly and move around the body.
•Redness and fever reflect Heat.
•Discharges often reflect Damp – yellow/green if Damp Heat and white/watery if Damp Cold.
•Nasal secretions may simply reflect Wind preventing the descent of Lung Qi (as in the common cold or allergies).
•Pus and swellings (including lymph nodes) may represent Phlegm or Damp.
•Susceptibility to infection reflects Depletion of Vital Substances (Blood, Qi, Yin or Yang).
•Susceptibility to infections that manifest on the Exterior may reflect depletion of Wei Qi in particular (and it is important to remember that the Kidneys (Essence) are the root of Wei Qi58).
The common cold
In the case of a familiar example, the common cold, the symptoms are conventionally attributed to infection by any one of a number of viruses. The common cold is unusual in that it is an infectious disease that is neatly described by a basic Organ syndrome, that is, either Wind/Heat or Wind/Cold invading the Lungs. This makes it a good example to use for an interpretation of energetics.
From a Chinese medicine perspective:
•Wind is present because the symptoms start suddenly, change rapidly and move to different parts of the body.
•Cold may predominate if secretions are watery and the patient feels cold.
•Heat may predominate if secretions are yellowish and fever is marked.
•Profuse secretions suggest impairment of Descent of Lung Qi due to Wind.
The Pathogenic Factors manifesting in the common cold
The Pathogenic Factors usually manifest on the Exterior. Exterior features are slight fever and sweating, headaches, sneezing and runny nose. There are often no features of a deeper Pathogenic Factor in a simple cold.
Occasionally, symptoms such as diarrhea accompany a cold. This might suggest possible involvement of the Interior with Depletion of Spleen Qi and Damp Coldor Damp Heat.
The origin of the Pathogenic Factors in the common cold
Most features are suggestive of an External origin. The common cold is highly infectious. Often, a particular type of cold that is “going around” will have a characteristic pattern of symptoms that will be experienced by most people who catch it. This suggests that the cold virus contributes a characteristic energetic property, and that the Pathogenic Factor is, at least in part, of External origin.
In addition, many people report that their symptoms follow exposure to particular extremes of climate such as cold wind. This suggests that climatic factors can also contribute to the nature of the Pathogenic Factor that is manifested.
Some features, however, may be suggestive of an Internal origin. Some people report that they benefit from having an occasional cold, particularly if the symptoms have been quite pronounced, but short-lived. This suggests that the infection has enabled clearance of existing internal imbalances, which may be seen as contributing to the severity of the symptoms. However, other people suffer from frequent or chronic cold symptoms that they are never able to shake off. They may suffer from colds even when one is not going around, and in energetic terms, this suggests that it may have become a manifestation of a lingering Pathogenic Factor of Internal origin.
Integrated energetic interpretation of the common cold
The above considerations can be summarized as follows:
•The different strains of the cold virus seem to have a characteristic energetic property that leads to manifestation of Wind/Heat or Wind/Cold on the Exterior and impairment of the Descent of Lung Qi.
•Deficiency of Wei Qi predisposes to development of the common cold. External climatic factors can contribute to the way in which the Pathogenic Factors are manifested.
•In health, the development of a cold may permit clearance of an internal imbalance, leaving the person in a more balanced state on recovery.
•If defenses are less strong, a cold may lead to a deepening imbalance with depletion of substances such as Qi, Yin and Yang, and continued manifestations of Lingering Pathogenic Factors.
Cellulitis
The condition of cellulitis offers a slightly different perspective from that of the common cold. Cellulitis is an infection of the subcutaneous tissues that usually follows a wound to the skin. It can cause a hot, red swelling of the skin, often around an abrasion on the leg or face, and which can spread. The patient feels unwell and feverish. The spread of the infection may be rapid and can lead to a red line tracking up the course of a lymphatic vessel. Lymph nodes draining the infected area can then become swollen and tender.
If untreated, cellulitis can become severe, and in some cases life-threatening, as infection leads to local death of tissues and septicemia (infection in the bloodstream). The cause of cellulitis is most commonly a skin commensal bacterium of either the Streptococcusor Staphylococcusfamilies.
Cellulitis is more common in older people and in people with chronic conditions such as diabetes.
From a Chinese medicine perspective:
•Wind is present because the symptoms start suddenly, change rapidly and move to different parts of the body.
•Redness, tenderness and fever all indicate that Heat is present.
The Pathogenic Factors manifesting in cellulitis
In cellulitis Wind Heat initially manifests on the Exterior, as indicated by an enlarging red area on the skin.
Symptoms such as lymphatic vessel involvement and malaise suggest that the infection has moved to a deeper level. As the cellulitis progresses, this suggests a progressively deeper level of pathology into the Interior.
The origin of the Pathogenic Factors in cellulitis
A wound is usually the trigger for cellulitis, and this means there will be local damage and Stagnation of Blood and Qi that might predispose to the generation of Heat. Other features of cellulitis are suggestive of Internal origin. Cellulitis is not a contagious disease. Even though superficial wounds can introduce germs, many people recover from these without developing cellulitis. Cellulitis is more likely to occur in people with depleted energy, such as elderly people and those with diabetes. It is also more likely to develop in regions of the body where there is poor circulation or local edema. In each of these situations, Deficiency of Qi and Blood, Blood Stagnation and internal Phlegm Damp and Heat are likely to be patterns that are already apparent in the patient.
Integrated energetic interpretation of cellulitis
The above considerations can be summarized as follows:
•Cellulitis is due to a spreading infection by the Streptococcus bacterium. It has features of Wind Heat invasion of the Exterior, but can progress to involve deeper levels in some people.
•Its features are predominantly those of a Pathogenic Factor of Internal origin, which suggests that a person who develops cellulitis is susceptible because they already have deficiency of Vital Substances such as Blood and Qi and Yin, together with Internal Heat.
Summary
The Chinese medicine interpretation of what conventional medicine would describe as infectious diseases demonstrates very effectively how the symptoms that form a bridge between the two viewpoints can ultimately enrich an understanding of the pathological processes at play.
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