The identified pathogens were retrospectively determined as relevant by an infectious disease specialist who integrated all available information. Etiologies were divided in four mutually exclusive groups: (i) bacterial (≥1 bacteria); (ii) viral (≥1 respiratory viruses); (iii) mixed (≥1 bacteria and ≥1 respiratory viruses); or (iv) no pathogen (including fungi, non-respiratory virus, bacterial contaminant including coagulase-negative staphylococci, propionibacterium, corynebacterium, colonizing oral and respiratory flora). For some children, rapid detection tests (Alere BinaxNOW Influenza A&B, Quidel QuickVue RSV Test) for respiratory syncytial virus (RSV) and influenza A/B virus were available. These results were additionally considered in forming the different groups.