The epidemiological studies cited have indicated that viruses are commonly associated with wheezing illnesses in populations, in individuals, and in time, but, unlike bacteria, are rarely found during asymptomatic periods. Viruses have been identified in up to 50% of wheezing illnesses and asthma exacerbations occurring in childhood, and in up to 20% of those in adults. In childhood the predominant organisms identified have been rhinoviruses. RSV and parainfluenza viruses, but coronaviruses have not been studied adequately. Wheezing appears to be more common during rhinovirus and RSV than other virus infections in children spontaneously presenting with respiratory infections to medical care, but all virus groups have been incriminated, and in general, wheezing occurs in upwards of 50% of viral infections in asthmatics followed prospectively. The few adult studies available show little difference between viruses in identification rates during wheezing, or propensity to result in wheezing. The predominant viruses change with age, and children with asthma seem to be more prone to symptomatic virus infections than other children, although the presence of atopy alone does not appear to be important. There are important gaps in our knowledge of the epidemiology of virus-associated wheezing attacks, and further prospective studies are required, using early investigation and sensitive methods for identifying rhinoviruses and coronaviruses, to study severe asthma in children and adults. It is hoped that the use of nucleic acid hybridization and newer antigen-detection techniques will improve the ability to identify difficult viruses such as coronaviruses and rhinoviruses in the future. The ability to identify subclinical infections and compare the ratio of subclinical to clinical infections in normal and asthmatic children would be useful but would require intense monitoring of both groups for an extended period (minimum 12 months to cover seasonal variation) with full virological studies every 2-4 weeks-a difficult and expensive task. Another important line of study would be to prospectively document indoor aeroallergen exposure and virus infections in the same individuals, and compare their importance as precipitants of acute severe asthma attacks. With a clearer understanding of the groups at risk for asthma attacks, and the factors which put them at risk and precipitate their attacks, effective preventive strategies will become more feasible.