Hygiene: immune system.

Pigs have known for a long time what doctors and scientists have just learned: Dirt is good for you. Well, at least some dirt. One recent Swiss study of 800 children found that kids who grow up on a farm are less susceptible to asthma and other allergies later in life than their city pals. The same holds true for kids raised with a pet in the home. In both cases, researchers concluded that children with more exposure to germs in dirt and animal waste develop stronger disease-fighting immune systems.

Within dirt and dust lie pieces of dead gram-negative bacteria--the same microorganisms that live in your gut and can cause illnesses such as meningitis and food poisoning. But fragments of the same bacteria also contain vital endotoxins, molecular tags that prime the immune system to identify harmful invaders. "If we're too hygienic and use all these antibacterial soaps, children aren't exposed to endotoxins," says Thomas Ball, a University of Arizona researcher. An immune system underexposed to endotoxins may not be able to combat unknown invaders later on like allergens (allergy-causing molecules). Could cleaning your room be hazardous to your health?