Lice-or "cooties," as children call them-are tiny wingless insects that live on humans and animals (their hosts) and suck their blood. Adult lice, about the size of sesame seeds, range in color from gray to brown. You can see them clearly with a magnifying glass.

Three types infest people: head lice, body lice, and pubic lice (crabs). Each bites your skin, causing extreme itching, and is usually otherwise harmless. But if you scratch, you can break the skin and get other skin infections. In rare cases, lice can pass along typhus, a serious bacterial infection.

Six to 12 million people worldwide are infected with head lice each year. The insects most often infest children ages three to ten from all backgrounds and living conditions, who then pass the bugs on to friends and family. Young girls and women tend to contract lice more often than boys and men do.

No one really knows how many people are affected in the United States, but the number of cases seems to be growing. Some head lice in the United States are becoming resistant to treatments, which contain mild amounts of insecticide. Perhaps a new strain of lice has arrived as well.

Some people feel ashamed when they get lice, but anyone can get them. Body-to-body contact or sharing personal items helps spread lice. For instance, crab lice can be passed through sexual contact or by lying on infected bedsheets. Head lice can move from an infected person's brush to another's hair. Body lice can spread by way of shared clothing.

You can do a lot to prevent the problem with simple measures. If you already have lice, you can treat the problem yourself. Over-the-counter insecticide shampoos, cream rinses, and lotions should rid you of the parasites. Other steps you can take at home will prevent new outbreaks.