Asthma prevalence is increasing in Western industrialized countries. The infectious theory of asthma onset hypothesizes that lower levels of IL-12 result in reduced TH1 stimulation and failure of the neonate to deviate from its TH2 bias at birth. Helminthic infections may influence TH2 immune responses and hence immune development. Although ecologic data would support a protective effect of parasitic infection on asthma development, this may be due to other exposures. To date, there is no conclusive evidence that parasitic infection protects against asthma develpment.