Add lice to the list of four-letter words that make people cringe. The wingless parasites are itchy and bothersome, and an infestation is often embarrassing to admit and challenging to conquer. But at least head lice have something going for them that a lot of other bugs don’t: They’re harmless.

“Ticks can transmit Lyme disease; mosquitoes transmit West Nile virus and malaria, among other things,” said Navy Capt. Kevin O’Meara, a physician and chief of pediatrics at Fort Belvoir Community Hospital in Virginia.

“But head lice – those guys are pretty benign,” O’Meara said. “They’re not dangerous. They’re just annoying.”

Three types of lice afflict humans. Pediculus humanus capitis, or the head louse, is common in childhood. Up to 25 percent of all school-age children will have head lice at some point, according to the Centers for Disease Control and Prevention, with anywhere from 6 million to 12 million cases reported each year in the United States in children ages 3 to 11.

As O’Meara explains, head lice don’t jump or fly. Instead, they crawl from one person’s head to another person’s. Less commonly, lice also can be transmitted through sharing personal items, such as towels at swim meets and pool parties, and pillows and other bedding at sleepaway camps.

Head lice live on the human scalp, where they eat meals of human blood and attach their eggs, or nits, tightly to the hair shaft. It takes seven to 10 days for the nits to hatch into baby lice, scientifically known as nymphs. The nymphs mature into adult lice anywhere from nine to 12 days after hatching, the CDC says. The sesame-seed-sized critters can live up to about a month on a person’s head.

“It may not be readily apparent that you have head lice,” O’Meara said. “Once you become sensitized to their saliva, you’ll start feeling very itchy. But that may be weeks after that first louse has crawled onto your scalp.”

O’Meara said it’s fine to see a health care provider for help getting rid of lice, but effective over-the-counter medications are also available. 

“Basically, you massage the medication into your hair, let it sit for at least 10 minutes, and then wash it out,” O’Meara said. “Generally, you don’t need to be treated again, but a lot of people do so after seven days because of fear of reinfestation.”