Hygiene hypothesis: wanted—dead or alive.

Linneberg in this letter offers a complementary explanation for the previous large increase and the recent small decline in asthma prevalence in several high income countries. Linneberg argues that the increase in allergic disease observed earlier may have been due to a lack of high allergen exposures in many westernized populations, and that a more recent increase in common environmental allergen exposure may have contributed to the recent decrease in prevalence by inducing immune tolerance. Int J Epidemiol. 2008 Dec 9. [Epub ahead of print]