The most common lice live in the hair on your head or pubic regions and sometimes in armpits and beards. They lay seven to ten eggs and form a large colony-up to many dozen lice-before the itching is bad enough to alert you.

Infestations-when lice invade your body-usually begin when females crawl from one person's hair to another human host. The lice lay eggs, called nits, on hair shafts, using a kind of glue to anchor them in place. These tiny football-shaped eggs take a week or two to hatch and a month to become adults. The louse climbs down the hair shaft, bites through your skin, and begins to feed on your blood. The itching gets worse as more lice mature.

Body lice don't live on your body but in the seams of your clothing. They lay their eggs there and visit your body only to feed. People who don't wash their clothing often enough can harbor lice.