Epidemiology of acute respiratory infections in young children from Thailand. A prospective evaluation of the epidemiology and presentations of acute respiratory infections in children younger than 5 years of age admitted to Children's Hospital Bangkok (1988 to 1989) was supported by the World Health Organization. There were 226 patients with the inclusion criteria: 1 to 5 years of age; duration of illness less than 2 weeks; no prior antibiotic therapy; and low socioeconomic status. The disease categories included: croup, 19 cases; bronchiolitis, 60 cases; and pneumonia, 147 cases. Pathogens isolated were: respiratory syncytial virus (40); parainfluenza III (1); influenza B (1); and adenovirus (1); bacterial infections were proved in 23 cases. No significant differences in clinical features between bacterial and viral pneumonia were found. Interstitial radiographic patterns were more common in viral pneumonia whereas alveolar patterns were more common in bacterial pneumonia. However, 91% of mixed radiographic patterns (interstitial and alveolar) in chest films were from viral pneumonia. 