Infection: friend or foe in the development of atopy and asthma? The epidemiological evidence.

There is increasing evidence from epidemiological studies to support the "hygiene hypothesis" with regard to the development of allergic diseases. This review will discuss the epidemiological findings relating to atopy and asthma, separately. The strongest arguments in favour of the "hygiene hypothesis" are the findings relating early life day care attendance to a significantly reduced risk of atopy and asthma, and the strong link between sibship size and the occurrence of atopy as confirmed by numerous studies. Furthermore, data relating serological immune responses to certain infections such as hepatitis A and Toxoplasma gondii suggest a role for such infections, or alternatively the lack of hygiene, to be operative in the inhibition of the development of allergic immune responses. The effects described seem to be stronger for atopy than for asthma. Furthermore, biological plausibility for such a hypothesis can be found from findings of recent in vitro studies and from animal experiments. Epidemiological studies attempting to identify single infections as being harmful or beneficial could fail, because it may be the total burden of microbial stimuli rather than a certain infection that directs the maturing immune system into a Th1-type T-helper cell polarized response. The timing of the exposure seems to be essential for promoting beneficial or harmful effects, and may provide a "window of opportunity".