It’s not necessary to treat all the children in a classroom, or all the people who live in the house, though anyone who shares a bed should get treated. We no longer use some of the more toxic insecticidal drugs that used to be employed, like lindane, which was a potential neurotoxin. The over-the-counter preparations that are available now are much safer, but there are concerns about resistance, so many people resort to prescription drugs. One that is commonly prescribed is topical ivermectin, a drug with what Dr. Jackson called “a strong safety portfolio,” but it can be expensive.

And there are many treatments out there that have no evidence to back them up. For example, Dr. Jackson said, there are clinics using heat on children’s hair, which may work under certain conditions, but there is no regulation of how it is being used “to quote unquote inactivate the lice,” she said. “I am skeptical.”

And then there are the “suffocation treatments,” from olive oil to mayonnaise, applied to the hair in hopes of killing off the insects without resorting to chemicals and insecticides. Some of these probably work some of the time but “all will work no matter how you place them if you don’t have lice,” Dr. Jackson said.

So diagnosis is key — someone experienced should look at the child’s head, looking for lice and not just for nits, since all kinds of debris in the hair can be mistaken for nits; everything from dandruff to dirt to drops of hair care products. And someone who knows the resistance patterns in your area should advise you about whether to use the over the counter preparations or the prescription medications.

The head louse life cycle takes about three weeks; the adult female, about the size and color of a sesame seed, lays about 10 eggs a day, and glues each one to a hair near the scalp. Eggs hatch in about nine days, warmed by body heat emanating from the scalp, and the larvae then leave the egg case (which remains glued to the hair, though it’s now empty) and go through several different developmental stages over the next nine to 12 days, before reaching adulthood and starting to lay their own eggs.

That’s why it can be helpful to treat more than once, usually after about nine days, if you are using a product that does not kill the eggs, or if you see live lice after a first treatment. And there are indeed some lice that are resistant to some insecticides, so it’s important to know what is common in your community.

Removing nits with a fine-toothed comb can be done after treating the child’s head, and usually is most effective when the hair is wet, though still not a short or simple process.