Figure 2. Etiology of severe pneumonia in children in developing countries.

Qualitative representation of the combined results of vaccine probe analyses (12, 113), studies of blood and lung aspirate cultures (14, 111, 153–155), and virus studies (128, 156) in predominantly HIV-uninfected children. Note that children might be infected by two or more lung pathogens simultaneously. Single-etiology studies and studies of children in the developed world suggest the undiagnosed portion might be attributable, at least in part, to respiratory viruses (adenoviruses, influenza viruses, parainfluenza virus, human metapneumovirus, cytomegalovirus, rhinovirus, enteroviruses, and coronaviruses), Pneumocystis jirovecii, Mycoplasma pneumoniae, Chlamydia trachomatis, and Chlamydia pneumoniae. Etiology differs substantially in young infants (112), HIV-infected children (157), and malnourished children (15) as well as by region; for example, S. aureus accounted for one-quarter of the cases of pneumonia in a large study in Chile (158).