The pathogenesis of both allergic diseases and asthma remains an unsolved mystery. Of considerable interest, and perhaps insight, into this problem are clinical observations that viral illnesses play an important role in the allergic diathesis. Conclusive evidence to establish a relationship linking viral illnesses to clinical allergy has yet to be established. Nonetheless, recent observations in this arena are intriguing to scientist and clinician alike. The following discussion reviews recent investigations that evaluate the relationship between viral illnesses and development of allergic disease and asthma.