If ever there was a time to look behind your kid's ears, this is it. It is back-to-school week for head lice.

The lice are back from summer camp and vacation resorts. They are ready to start kindergarten and first grade, gearing up for louse epidemics that bloom in Washington area schools during the fall and winter months.

Squeaky-clean hair and a new wardrobe are no protection. Though for unknown reasons black children almost never get head lice, the head louse observes no other boundaries. Private or public school, long hair or short, dirty or clean -- it is all the same to it.

The world today is at war with head lice. Dr. Dennis D. Juranek, deputy director of the Parasitic Diseases division of the Center for Disease Control, said the beasts are at the peak of one of their 20- to 30-year cycles. The last two peaks came at the ends of world wars, and were explained away by troop movements and social upheaval. Now, said Juranek, the bug is back and no one knows why.

Head lice seem to prefer children. They most often infest the scalps of kindergarten pupils and those in early elementary school grades. The problem drops off in the higher grades, especially beyond junior high school, according to Juranek, although adults can get lice.

Lice do not have a known seasonal cycle, but school epidemics usually surface in the late fall and winter months. Juranek postulated that a few children may pick up lice in the summer, perhaps at camp or during foreign travel. If they are not found and treated in September, the lice "may smoulder until 10 to 15 percent of the kids are infested," he said. Like animal lice, head lice seem to flourish in thick hair and the crowded conditions of the winter months.

The head louse is a gray insect about the size of an ant, which lives in human hair, getting its meals by sucking blood from the scalp. Though it carries no known diseases, the bites are itchy. A single female louse can lay up to 400 eggs on individual hairshafts during its 30-day life, Juranek said.

Adult lice sometimes can be seen crawling behind the ears, under bangs or at the nape of the neck, Juranek said they are difficult to find because a child usually has only one or two adult lice on his scalp at a time. Parents and doctors therefore have to look for eggs, or "nits" to prove the louse's presence.

The one-millimeter-long nits look like flecks of dandruff, they are firmly cemented to the hair, Juranek said. Laid by the female close to the scalp, they are carried along as the hair grows, about one-quarter inch a week. Some of the eggs hatch within a week, producing "nymphs" which become mature within ten days. The rest are inactive, but remain attached to the hair.

Ordinary shampoo does nothing to head lice, but they are easy to get rid of with safe louse-killing shampoos, Juramel said. He recommended that a child suspected of having lice be checked by a doctor, who can prescribe shampoo or can suggest a product.

He said the Center for Disease Control advises schools that pupils be allowed to return the day after they use the shampoo. The shampoo does not kill unhatched lice, so a second application is necessary in a week to kill any nymphs that may have hatched. But the child will not spread lice if he has shampooed once, he said, because new nymphs are immature and cannot mate.

In the past, schools have demanded that parents remove inactive nits from children's scalps and have fumigated classrooms, but such measures are unnecessary,Juranek said.

Lice spread in two ways: either by crawling from one person to another via a "hair bridge" or by traveling on objects such as brushes, hats, seats and carpets. Juranek suspects person-to-person transmission is more inportant, because it helps explain why small children are the major victims.

Five and six year-olds "climb all over one another" and sit head-to- head playing games in classrooms, he said. But by the third or fourth grade, children learn to keep more of a distance from each other, making it more difficult for lice to spread.

"The louse doesn't have wings and it doesn't have jumping legs. It has to slowly crawl" from person to person, Juranek said.

Examining thousands of children in schools with head lice epidemics, Juranek and his team have squelched a number of myths. Lice are not more common in hot climates or poor neighborhoods, and they show no preference for long hair or dirt. Girls are afflicted more often than boys, but short-haired boys and girls get lice just as often as long-haired ones. Black children seem to be protected, but Juranek said no one knows whether this is because of hair differences.

Many times, Juranek said, people over-react in campaigns against head lice, which cannot hatch or survive long away from the warmth and food the scalp provides. Except for putting an affected child's bed linen and recently-worn clothes through a hot washer or dryer, extensive disinfecting is unnecessary.

"When people get these things they don't know when to stop cleaning," said Juranek. They put hundred-dollar suits through the dryer and wash their shoes with Lysol, often causing skin irritation from the disinfectant chemicals that becomes a bigger problem than the original lice.