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.