Head lice (Pediculis humanus capitis) have been a head-scratching nuisance for millennia. The wingless, six-legged, blood-sucking insects make their homes on the scalps of humans because food is plentiful and the environment warm and moist. In the United States, between 6 million and 12 million children ages 3 to 11 get lice each year [source: CDC].
Many scientists look at how resistant the bug has become to various treatments. John Clark, a professor of environmental toxicology and chemistry at the University of Massachusetts at Amherst, found that some lice have mutated into "super lice." Clark says one specific mutation allows head lice to resist two insecticides, pyrethrins and pyrethroids, which are found in many over-the-counter treatments. Clark estimates that 90 percent of head lice in the United States have the mutations [source: Clark].