Head lice are parasitic insects that feed exclusively on human blood several times a day, states the Centers for Disease Control and Prevention. They spend their entire lives on the human scalp, mostly behind the ears and on the neckline, and spread to other humans through direct head-to-head contact.
Head lice cause itching, sores on the head due to scratching, irritability and loss of sleep to the sufferer but do not spread disease, states the CDC. Treating head lice at home with lice shampoo and spacing two treatments seven to nine days apart usually works to clear an infestation, states the American Academy of Dermatology. When this does not work, dermatologists recommend using malathion lotion, benzyl alcohol lotion and lindane shampoo, which are more effective.