Causes

A head louse is a tan or grayish insect about the size of a strawberry seed. It feeds on human blood from a person's scalp. The female louse produces a sticky substance that firmly attaches each egg to the base of a hair shaft no more than 3/16 inch (5 millimeters) from the scalp.

The louse life cycle

A louse goes through three stages:

• Eggs that hatch after six to nine days.
• Nymphs, immature forms of the louse that become mature adults after nine to 12 days.
• Adult lice, which can live for three to four weeks. The female louse lays six to 10 eggs a day.

Transmission

Head lice crawl, but they can't jump or fly. Transmission of a head louse from one person to another is often by direct head-to-head contact, often within a family or among children who have close contact at school or play.

Indirect transmission is uncommon, but lice may spread from one person to another by items such as:

• Hats and scarves
• Brushes and combs
• Hair accessories
• Headphones
• Pillows, towels and upholstery

Indirect transfer could also occur among items of clothing stored together. For example, hats or scarves hung on the same hook or stored in the same school locker could serve as vehicles for transmitting lice.

Household pets, such as dogs and cats, don't play a role in spreading head lice.