8. Epidemiology of infectious diseases

Questions

Contrast the main routes of transmission of infectious disease

What are the differences between endemic and epidemic outbreaks?

Epidemiology is ‘the study of the occurrence, distribution and control of disease in populations’. The risk of infection depends upon more than simply an individual’s susceptibility; it is also affected by the level of disease within the population, the degree of population mixing and ‘herd immunity’, as well as specific features such as the communicable period, route and ease (infectiousness) of transmission.

Route of transmission

For an infectious agent to persist within a population there must be a cycle of transmission from a contaminated source, through a portal of entry, into a susceptible host and on again (Fig. 3.8.1).

Fig. 3.8.1 Routes of transmission of infectious agents.

Direct transmission. This is the most common and important route. It involves all forms of physical contact between humans, including sexual transmission, faecal–oral spread and direct respiratory spread via large droplets.

Vector-borne transmission. Infection is mediated by arthropods or insects; it is mechanical if the vector is simply a source of contamination, but biological if the vector is necessary for the multiplication or maturation of the infectious agent.

Vehicle-borne transmission. This describes spread from all contaminated inanimate objects. Vehicles include clothing, food, water, surgical instruments and also biological substances such as blood and tissues.

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