Pediculosis capitis is the medical term for a head lice infestation. It describes a condition where tiny parasitic insects called head lice live on the human scalp, feeding on blood several times a day and causing itching that can range from mild to intense. An estimated 6 to 12 million infestations occur each year among children aged 3 to 11 in the United States alone, making it one of the most common parasitic conditions in school-age kids.
What Head Lice Are
Head lice are small wingless insects whose only known hosts are humans. They cannot fly or jump. An adult louse is roughly the size of a sesame seed, with females measuring about 3 to 4 mm long and males slightly smaller. Their eggs, called nits, are much tinier at just 0.8 mm by 0.3 mm, and are cemented to individual hair shafts close to the scalp where body heat keeps them warm enough to develop.
Lice depend entirely on human blood to survive. Both immature lice (nymphs) and adults feed multiple times per day by piercing the scalp with small mouthparts and injecting saliva to prevent clotting. Without a human host, a louse dies within one to two days because it cannot get the blood meals it needs.
Life Cycle on the Scalp
The life cycle of a head louse moves through three stages: egg, nymph, and adult. A female louse lays eggs directly on hair shafts, typically within a few millimeters of the scalp. Those eggs hatch in about one week, though the range is six to nine days. The newly hatched nymph looks like a miniature adult, roughly the size of a pinhead, and goes through three molts over the next seven days before reaching maturity. Once fully grown, an adult louse can live up to 30 days on a person’s head, and a female may lay several eggs per day during that time.
This timeline matters for treatment. Because most treatments kill live lice but not all eggs, a second round of treatment is typically needed about a week later to catch newly hatched nymphs before they mature and start laying eggs of their own.
How It Spreads
Head-to-head contact is by far the primary way lice spread. When two people’s hair touches, a louse can crawl from one head to the other. This is why infestations cluster among young children who play closely together and among family members who share a household.
Spread through objects like hats, combs, brushes, pillows, or stuffed animals is possible but much less common. Because lice die quickly without a host, items that haven’t been in contact with an infested person’s head recently pose very little risk. Extensive cleaning of a home is generally unnecessary. Washing recently used pillowcases and towels in hot water and avoiding shared hair accessories for a few days is usually sufficient.
Why It Itches (and Why It Takes Weeks)
The hallmark symptom of pediculosis capitis is itching, but the itch isn’t caused by the bite itself. It comes from a hypersensitivity reaction to proteins in the louse’s saliva, which gets injected into the scalp during each feeding. Your immune system gradually recognizes these proteins as foreign and mounts an allergic response.
Here’s the surprising part: if you’ve never had lice before, itching may not develop for four to six weeks. That’s how long it takes your body to become sensitized to the saliva. During that window, an infestation can grow and spread to others without the host feeling a thing. People who’ve had lice before tend to start itching much sooner because their immune system already recognizes the allergen.
Beyond itching, you might notice a tickling sensation of something moving in the hair, difficulty sleeping (lice are more active in the dark), or small red bumps on the scalp, neck, and behind the ears.
How an Active Infestation Is Diagnosed
The definitive diagnosis requires finding a living louse on the head. This sounds straightforward, but lice are fast, light-shy, and surprisingly hard to spot during a casual look. Combing through wet hair with a fine-toothed louse comb is about four times more effective than visual inspection alone and takes roughly half the time.
Finding nits does not necessarily mean you have an active infestation. Nits can remain glued to hair shafts long after an infestation has ended, and without a microscope, it’s difficult to tell whether a nit is viable or empty. Nits found more than a centimeter from the scalp have almost certainly already hatched or died. Even nits close to the scalp are only a modest predictor of active infestation. Many people, including school nurses, have trouble distinguishing between current and past infestations when relying on nit detection alone.
Treatment and Resistance
Over-the-counter lice shampoos containing pyrethrins or synthetic pyrethroids were the go-to treatment for decades, but resistance has become a significant problem. Genetic mutations that make lice resistant to these products have been found in lice populations across the United States, Canada, France, Argentina, and several other countries. In some studied populations, the resistant gene appears in over 98% of lice tested. If you’ve used an over-the-counter product correctly, followed the directions, and still see live lice a day or two later, resistance is the likely explanation.
When standard products fail, prescription options are available. These work through different mechanisms that bypass the resistance problem. In clinical trials comparing treatments for difficult cases, oral medications cleared lice in about 95% of patients by day 15, compared to roughly 85% for a commonly used prescription lotion. Your pharmacist or doctor can help you choose an appropriate next step based on what’s available in your area.
Regardless of the product used, a second treatment about seven to nine days after the first is standard practice to kill any nymphs that hatch from surviving eggs. Thorough combing with a fine-toothed nit comb after each treatment helps remove both dead lice and remaining eggs.
Complications From Scratching
Head lice themselves don’t transmit diseases, but persistent scratching can break the skin on the scalp, creating openings for bacteria. This can lead to secondary infections like impetigo (crusty, honey-colored sores) or deeper skin infections. Signs to watch for include increasing redness, warmth, oozing, or crusting on the scalp, particularly behind the ears or at the nape of the neck. Repeated or aggressive use of lice treatments can also irritate the scalp and contribute to skin breakdown, so following product instructions carefully and not over-treating is important.
Who Gets It and Who Doesn’t
Head lice infestations have nothing to do with hygiene. Lice are equally happy on clean or dirty hair. They affect all socioeconomic groups and all hair types, though prevalence varies across populations. In the U.S., infestations are less common among Black children, likely because the oval cross-section of tightly coiled hair makes it harder for lice (which evolved to grip round hair shafts) to attach their eggs.
Girls get lice more often than boys, probably because of play styles that involve more head-to-head contact and more sharing of hair accessories. Adults can get lice too, though it’s far less common than in children. Parents and caregivers of infested children are the most likely adult group to pick up lice, usually through close contact during daily caregiving.