More evidence for the hygiene hypothesis, from Rat study shows dirty better than clean:
[T]he wild mice and rats had as much as four times higher levels of immunoglobulins, yet weren’t sick, showing an immune system tuned to fight crucial germs, but not minor irritants, Parker said. He said what happened in the lab rats is what likely occurs in humans: their immune systems have got it so cushy they overreact to smallest of problems.“Your immune system is like the person who lives in the perfect house and has all the food they want, you’re going to start worrying about the little things like someone stepping on your flowers,” Parker said.
Challenged immune systems — such as kids who grow up with two or more pets — don’t tend to develop as many allergies, said Dr. Stanley Goldstein, director of Allergy & Asthma Care of Long Island.