At least 15 different virus groups, three Mycoplasma species, one rickettsia, three Chlamydia species, and one protozoan parasite have been associated with pneumonia syndromes in children. The overall importance of these agents is not measured simply by their incidence. Some agents, although they are fairly common, generally give rise to relatively mild illness; others encountered less frequently characteristically cause serious disease. In Table 230.1, the major agents causing disease in various age groups are presented according to their overall frequency, their typical degree of severity, and their usual mode of access to the lung.

Respiratory syncytial virus (RSV) is the most common cause of pediatric pneumonia, particularly if it is associated with bronchiolitis. Although infection with this virus is fairly common in all age groups, lower respiratory tract involvement is especially prominent in infancy.

The three parainfluenza viruses (types 1, 2, and 3) are second only to RSV as causes of lower respiratory tract disease in infants and younger children. Of these agents, parainfluenza virus 3 occurs most frequently in pneumonia; infection by parainfluenza viruses 1 and 2 generally produces laryngotracheitis.

Human metapneumovirus (hMPV) is a recently discovered respiratory virus that appears to be a significant cause of lower respiratory tract infection in infants and children. It is classified in the same family (Paramyxoviridae) and subfamily (Pneumovirus) as RSV. Its symptoms appear to be similar to those of RSV and influenza. It is estimated to be the cause of 12% of lower respiratory tract infections in children younger than the age of 5 years.

Influenza viruses A and B are not as prevalent overall as RSV, the parainfluenza viruses, and hMPV, but during periods of epidemic spread, they may become predominant isolates in ambulatory and hospitalized children with lower respiratory tract diseases.

Sometimes adenoviruses are isolated in children with pneumonia and with pertussis syndrome. Although the overall frequency of these viruses is somewhat less than that of the other common respiratory viruses, numerous fatal illnesses have been reported. Of the 31 known adenoviruses, types 1, 2, 3, 4, 5, 7, 8, 11, 21, and 35 have been associated clearly with pneumonia. Some degree of lower respiratory tract involvement by rhinoviruses is indicated by their documented role in exacerbations of asthma and bronchitis. Among the enteroviruses, primary virus pneumonia has been documented best with coxsackieviruses A9 and B1, although coxsackieviruses A16, B4, and B5 and echoviruses 9, 11, 19, 20, and 22 also have been reported.

The human coronaviruses HCoV-O43 and HCoV-229E have been implicated as causes of pneumonia in a few seroepidemiologic studies, but recovery of these agents in tissue culture has been rare. Two other newly described coronaviruses, HCoV-NL63 and HCoV-NH (possibly the same species), have been shown to be relatively common causes of acute respiratory illnesses in children in the Netherlands and in Connecticut. Their role in viral pneumonia is not yet defined.

Another newly discovered coronavirus is responsible for an important entity called severe acute respiratory syndrome, otherwise known as SARS. This disease first appeared in southern China in November of 2002 and rapidly spread to 29 countries. More than 8,000 cases were reported worldwide, resulting in more than 900 deaths. The epidemic now appears to have been controlled through an extraordinary global public health effort. Lower respiratory infection by the SARS coronavirus (SARS-CoV) frequently leads to acute respiratory distress syndrome with respiratory failure. Younger children appear to be affected much less severely than are teenagers and adults.

Pneumonia is the most common serious complication of measles. On careful radiographic study, at least one-half of all patients with routine cases of measles have pulmonary infiltrates early in the illness, a finding suggesting a viral rather than a bacterial cause. Secondary pneumonia in measles results from the common bacterial pathogens, particularly Streptococcus pneumoniae and Staphylococcus aureus. Progressive, fatal, primary measles pneumonia (Hecht giant cell pneumonia) can occur in patients with cell-mediated immunodeficiency, hematologic malignancy, or acquired immunodeficiency resulting from human immunodeficiency virus (HIV) infection. The characteristic measles rash often is absent.

Viruses that may attack the lungs by hematogenous spread include varicella-zoster virus (VZV), Epstein-Barr virus (EBV), rubella, cytomegalovirus (CMV), herpes simplex viruses (HSV), and HIV. Rubella, CMV, and HSV may cause interstitial pneumonia in the infant infected congenitally or perinatally. CMV and VZV are causes of life-threatening pneumonia in immunocompromised hosts. Pneumonia has been noted in adolescents with infectious mononucleosis. Pulmonary infiltration is also a component of the fatal X-linked lymphoproliferative syndrome that is caused by EBV. Pulmonary lymphoid hyperplasia (lymphoid interstitial pneumonitis) is the most frequent cause of pneumonia in pediatric acquired immunodeficiency syndrome (AIDS). Whether this subacute to chronic condition is the direct result of pulmonary infection by HIV or is triggered by concomitant viral infection (e.g., by EBV) remains unclear.

Of the 15 known Mycoplasma species that can infect humans, only M. pneumoniae is a well-established cause of atypical pneumonia. In children younger than 2 years old, infection is common, but pneumonia is an unusual development. In children older than 5 years, M. pneumoniae is the most common cause of pneumonia. Genital mycoplasmas—Ureaplasma urealyticum and M. hominis in particular—have been associated with infant pneumonia acquired congenitally and perinatally.