Although pneumonia is the principal clinical sign in children with acute lower respiratory infections (ALRI), researchers have an incomplete understanding of pneumonia etiology. This is largely because of the lack of effective bacterial pneumonia diagnostics, which makes it difficult to confirm the role of a particular bacterial causative agent, since so many healthy children carry Hib and S. pneumoniae. Based on a model developed by the CDC, S. pneumoniae is thought to account for the majority of deaths in children under age 5 (Figure 2). The pathogenesis of acute respiratory disease also is poorly understood, in part because of the paucity of developing-world autopsy data. Also requiring more study is the relative importance of viruses and bacteria in causing illness.

FIGURE 2 Annual deaths of children under age 5 by main causes. For the purpose of this chart, one cause has been allocated for each child’s death, in practice, children often die of multiple causes, and malnutrition is a contributory cause in approximately one-third of all child deaths. Measles deaths are sometimes ascribed to acute respiratory infection, as a severe case of measles renders a child highly susceptible to other infections. Pneumonia is often the ultimate reason for a death for which measles is primarily responsible. SOURCE: Adapted from UNICEF, 1990.