O'Brien KL, Baggett HC, Brooks WA, et al. - Through a multi-site, international case-control study conducted in nine study sites in seven countries (Bangladesh, The Gambia, Kenya, Mali, South Africa, Thailand, and Zambia) of 4,232 cases, researchers determined causes of pneumonia in young African and Asian children, using novel analytical methods employed to clinical and microbiological findings. The discovery of respiratory syncytial virus (RSV), parainfluenza virus, human metapneumovirus, influenza virus, Streptococcus pneumoniae, Haemophilus influenzae type b (Hib), H influenzae non-type b, and Pneumocystis jirovecii in nasopharyngeal and oropharyngeal (NP-OP) specimens was related to case status. The etiology analysis concluded that viruses accounted for 61·4% of causes, whereas bacteria accounted for 27·3% and Mycobacterium tuberculosis for 5·9%. In very severe pneumonia cases vs severe cases, viruses were less prevalent and bacteria was more. Of all pathogens, RSV had the highest etiological fraction. Human rhinovirus, human metapneumovirus A or B, human parainfluenza virus, S pneumoniae, M tuberculosis, and H influenzae each estimated for 5% or more of the aetiological distribution. Variations in etiological fraction by age for Bordetella pertussis, parainfluenza types 1 and 3, parechovirus–enterovirus, P jirovecii, RSV, rhinovirus, Staphylococcus aureus, and S pneumoniae, and variations by severity for RSV, S aureus, S pneumoniae, and parainfluenza type 3, were noted. The leading ten pathogens of each site estimated for 79% or more of the site's etiological fraction. Thus, a small set of pathogens accounted for most cases of pneumonia needing hospital admission. Moreover, preventing and treating a subset of pathogens could mainly influence childhood pneumonia outcomes.