Head lice. Artwork showing a close up of a head louse egg, or nit, on a hair shaft (top) and a person's hair being combed with a nit comb (bottom). Adult female head lice lay between 80 to 100 eggs during their life. These hatch after a week and develop rapidly, with successive generations arising every three weeks. Adults live in the hair on the head and suck blood from the scalp, causing intense itching. The itching provokes scratching, which can lead to scalp inflammation, or dermatitis. Treatment is with medicated shampoo and combing.