Head lice are about the size of a sesame seed, and usually light brown. Females live an average of 27 days; males 16 days. The female lays five to eight eggs each day, for a potential lifetime total of 300. The yellowish white eggs, or nits, are only one millimeter, or 4/100ths of an inch, and cling to a hair shaft. The colorless newly hatched nymphs are the size of the period at the end of this sentence. They feed immediately. After three molts in a week, they become adults. The bite of the louse causes itching and infection because of our allergic reaction to a protein in the louse's saliva. Some people are immune; others gain immunity after many bites. They do not fly, as some people believe, but are capable of running as fast as nine inches a minute. Their nickname: cooties. Scientists believe that lice inhabited the bodies of our hairy ancestors and, as we became less hirsute, some moved upward to our tresses. They can be fussy. "On some of us, they do very well," says University of Miami Medical School Prof. David Taplin. "But a lady on our field research team kills every louse you put on her."

The researchers are trying to find out why.

Because head lice are not a "reportable condition," the federal Centers for Disease Control doesn't keep statistics on the number of cases. However, CDC medical entomologist James Stewart says that autumn is the time of year cases of head lice are most prevalent.

"With schools starting up, and kids sharing hats and combs, this is when we get the most calls about head lice."

In the Washington area, school systems are generally reporting fewer cases this year than in years past, but no one is denying that head lice exist. "We always have our fair share," says Dr. Florence Fenton, supervisor of Health Education and Health Services for Prince George's County Public Schools.

In Montgomery County, 22 of the 100 elementary schools have reported at least one case since the beginning of the school year, although a spokesperson characterized that as "way down from last year at this time." Both the District and Arlington County public schools also report fewer cases.

Seriousness of the problem nationally, say parents and representatives of the National Pediculosis Association, varies between schools and states.