Head lice are parasites that can live in human heads. A widespread occurrence, especially in children, head lice are not harmful, but they do make the scalp extremely itchy and uncomfortable, sometimes resulting in scalp sores. The lice, or one louse, can jump from one head to another by contact. Although you should be aware that even wearing infected clothing, hats, hair ties, using infested brushes and combs, or even lying on a couch or pillow that has recently had contact with an infested head, will lead to transmitting the parasites. Once latched on to their new host they will lay eggs, otherwise known as the nit, in the hair, aiming to multiply as fast as possible. Gestation takes one week for the eggs to hatch and the entire lifespan of a louse from hatch to death is up to 30 days.
You'll want to be rid of them before 30 days though, because if you don't treat your head lice the cycle will continue, plus, it's extremely itchy. Can buy over the counter chemical shampoos and solutions that kill the lice before you comb and wash them out, or there are many non chemical-based products available, especially if you are treating young children.