Last night while driving home I heard Act II of This American Life episode 404 about a guy with terrible allergies and asthma who learned that those diseases basically don't exist in the developing world. The reason for this, he learned, is a parasite, specifically hookworm. According to the program, having a hookworm calms your immune system response so you don't develop that condition where your immune system is trying to kill you. (my exaggerated non-scientific language, but still, you get the point)
So this guy goes to Africa and walks around barefoot in yucky places to intentionally infect himself with hookworm. He returns to Santa Cruz and the next time allergy season comes around he has no issues. His eyes don't swell shut. He is not sent to the hospital because of severe asthma attacks. He is cured. He drives down the highway with his windows rolled down shouting for joy.