Ironically i recently read a study that explored the theory that our allergies are caused by the modern lack of intestinal parasites. They release a moderate immune suppressant into the host organism, helping to desensitize the immune system so it doesn't react as easily to non threats like random pollen.
Yeah, anecdotal, but my buddy didn't get hayfever the spring he had a tapeworm. Also lost a bunch of weight. He says it was the best thing to ever happen to him.