The occurrence of respiratory viruses in persons with asthma and COPD must be viewed against the behavior of the viruses in persons without these conditions. There are multiple agents involved, and reinfections with the same virus are frequent. Infections and illnesses generally decrease in frequency with increasing age. Thus, it can be expected that infections among children, with or without asthma, will be more frequent than among adults in general. Evaluation of studies of viral activity in persons with asthma and COPD should differentiate between increased susceptibility and production of exacerbations. The latter may simply indicate the potential for greater severity, but not a higher frequency, of infections. Whatever viruses are circulating at a particular time can produce exacerbations; the studies are most conclusive for asthma in children, probably because infections in general are more common in this age group. Data also suggest increased susceptibility, although results are less clear than for exacerbations. Therefore, persons with chronic respiratory diseases should be considered a risk group for current and developmental vaccines and antiviral medications.