What are lice? Head lice, or Pediculus humanus capitis, are extremely contagious insect parasites that are essentially harmless. Unlike their cousin, body lice, or Pediculus humanus humanus, head lice don’t carry diseases. The microscopic insects live in your hair, close to your scalp. Head lice must feed off another living body in order to survive. Their source of food is human blood, which they get from your scalp. Head lice can’t fly, aren’t airborne, and can’t live in water very long away from their host. In fact, they cling to hair strands for dear life when you bathe. But where do they come from in the first place?

Geographic origins Human head lice are categorized into clades based on their genetic makeup. A clade is a group of organisms that are not genetically identical to one another, but share a common ancestor. The clades of human head lice, named A, B, and C, have different geographic distribution and varying genetic characteristics. According to the Journal of Parasitology , Clade B head lice originated in North America, but migrated to farther reaches of the world, including Australia and Europe.

Human evolution and lice Head lice are thought to have separated from body lice, a similar yet distinct species, a little more than 100,000 years ago. The discovery of genetic differences between head and body lice supports theories that this time period is when people began wearing clothing. While head lice remained on the scalp, body lice mutated into a parasite with claws that can grab on to the smoother fibers of clothing rather than needle-thin hair shafts.

How are lice transmitted? Head lice are transmitted from one host to another through close personal contact. For the most part, this means that a non-infested person would have to be in head-to-head contact with an infected person. Sharing combs, brushes, towels, hats and other personal items can hasten the spread of head lice. The louse travels by crawling. In rare cases, head lice can crawl onto a person’s clothing and on to another person’s hair and scalp, but this must happen quickly. Lice can’t live more than a day or so without nourishment.

Misconceptions Having a case of lice can be embarrassing. A common misconception about head lice is that it is a sign of poor personal hygiene. Some even believe that it affects only people of lower economic status. These ideas can’t be farther from the truth. People of all genders, ages, races, and social classes can catch head lice.