On a dare, teen advances medical science.

A 16-year-old daredevil inadvertently demonstrated the incubation period of a common roundworm-caused disease after she swallowed an earthworm at a friend's behest. The earthworm harbored larvae of the parasite, her doctors later concluded.

It took about 4 weeks for the young woman to develop a cough, wheeze, nausea, fever, and facial swelling.

Roundworms of the genus Toxocara are common in dogs and cats and annually infect about 10,000 people in the United States, mostly children who play in yards fouled with pet feces. Because infected kids are usually in frequent contact with the contaminated dirt, the parasite's incubation period in people has been difficult to assess.

According to a report in the February Pediatrics, dogs had frequently played in the friend's yard from which the ingested earthworm was taken, and the patient had no other potential exposure to roundworm larvae. LeAnne Fox of Children's Hospital in Boston and her colleagues, who diagnosed the cause of illness, successfully treated it by administering a 10-day course of an anti-worm drug.

Most Toxocara infections in people cause no symptoms and go away without treatment, but in the lungs, the parasite can cause problems that mimic pneumonia or asthma. The worms can also affect the eye and liver.--B.H.