Head lice are small biting insects that are spread by direct contact. This means that the cause of a head lice infestation is sharing items like hairbrushes that are infested with life, or direct head-to-head contact with an infested person.
Head lice are a specific species called Pediculus humanus capitis and are different from the species of lice that can live on other parts of the body. They attach their eggs to human hairs for incubation. They cannot jump or fly, so avoiding contact with infested items or people prevents transmission. Lice often crawl onto hats, hairbrushes, scarves, pillows and other common items. When a new person uses those items, the lice crawl off and infest them.