“I am as gentle as I can be, much more gentle than sex is for them,” she said, “but even the slightest touch and they bleed.”

Gentleness is part of Dr. Kjetland’s nature. A 49-year-old stepmother of five, she watches like a mother over the girls in her study. She ordered that extra rooms be built where they can cry if they test positive for H.I.V. She tries to make sure the boys in their schools never realize she chooses only sexually active girls. And she has KFC delivered, since it is a treat for girls who often have only cornmeal mush to eat for days on end.

Though trained in Norway, she has spent most of her life in Africa, growing up in Tanzania as the eldest of a missionary couple’s six children, attending prep school in South Africa, and, after college and medical school in Norway, doing graduate work in Malawi and Zimbabwe.

An estimated 200 million Africans have had schistosomiasis. Although it is rarely fatal, the bleeding it causes in children can lead to anemia, stunted growth and learning problems. It is caused by tiny worms that live in freshwater snails and emerge with pointed heads that can penetrate the skin of people collecting water or washing clothes.

Once inside, the worms mate, with the female living in a cleft in the male’s body “like a hot dog in a bun,” Dr. Kjetland said. Most nest in the urinary tract — bloody urine is the classic symptom — but a portion end up in the vagina, creating “sandy patches” of damaged tissue and calcified eggs.

Studies by Dr. Kjetland in Zimbabwe and South Africa and by Dr. Jennifer A. Downs of Weill Cornell Medical College in Tanzania have shown that women with the patches are about three times as likely as their neighbors to be infected with H.I.V.

A gold standard study to prove the connection would be both impractical and unethical: Researchers would have to divide hundreds of infant girls into two groups, give half deworming drugs and half placebos, wait until they were perhaps 20 years old, and see how many had H.I.V. No ethics board would approve placebos under those conditions.