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There are many types of lice, the head louse, or Pediculus humanus capitis, is a parasitic insect that can be found on the head and, more rarely, the eyebrows and eyelashes of people.
There are three stages of the lice life cycle: egg, nymph and adult.
Head lice have six claws designed for them to crawl from hair strand to hair strand. These claws allow them to move from head to head.
A head-lice infestation occurs when a female adult louse moves to a new head and lays eggs. When those eggs hatch, the lice will most likely stay on that head throughout the entire lice life cycle. Unless the head is treated and all lice and eggs are killed, the lice infestation will continue for however long the lice can live.