Parasites and asthma/allergy: what is the relationship?

Asthma prevalence is increasing in Western industrialized countries. The infectious theory of asthma onset hypothesizes that lower levels of IL-12 result in reduced T(H)1 stimulation and failure of the neonate to deviate from its T(H)2 bias at birth. Helminthic infections may influence T(H)2 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 development.