It is well established that intestinal parasites dampen mammalian immune reactions. But in a surprise result, scientists have found that another kind of parasite – the body louse – does too. That means the epidemic of allergic disorders in modern, urban people might be due to our having rid ourselves of lice and worms.

The "hygiene hypothesis" holds that our immune systems evolved to compensate for continual infections with parasitic gut worms, which secrete chemicals that reduce our immune responses. People who are now worm-free have overreactive immune systems, which can lead to asthma and autoimmune disorders.