Like fleas, lice are parasitic bugs that live on their hosts, but they're even more specialized — not only do certain lice target certain animals, but they even target certain parts of certain animals. Take the three species that bite people, for example: head lice (pictured), body lice and pubic lice. Each one preys on its own distinct niche in the human body, often swarming in one area while virtually absent everywhere else.
The problem of head lice in schools has given that species more notoriety, but body lice are the only ones that spread disease. Typhus, trench fever and relapsing fever can all be transmitted by body lice, although in the U.S. they're mostly found among homeless people or others who don't have access to regular bathing or changes of clean clothes.