The occurrence of allergic disease is skyrocketing, and can be quire high in western societies. Allergies are specific and reproducible undesired and unpleasant immune responses that are triggered by naturally occurring substances such as foods, pollens or other influences in our surroundings. Overwhelming evidence from various studies suggests that the “hygiene hypothesis” explains most of the allergy epidemic.

http://fooddrugallergy.ucla.edu/body.cfm?id=40