Worm war.

Peter Emjor from Nigeria, Africa, tried to stay calm as a stringy worm was pulled from his foot. Like many people from his village, Emjor had been infected by a parasite known as the Guinea worm.

How does such a worm get into a person's skin? The worm starts its life as a pinhead-size larva living inside a water flea. When villagers drink unfiltered water from a pond, they also gulp down the worm-infected fleas, says Ernesto Ruiz-Tiben, technical director of the Carter Center's Guinea Worm Eradication Program. As a person digests the water fleas, the worm larvae get released into the person's stomach tissues. There, the developing worms absorb nutrients from the victim's tissues.

A female worm can grow to it's full length of 1 meter (3 feet), while the infected person is unaware of the stringy intruder. The full-grown worm travels through the person's subcutaneous tissue--a deep skin layer containing fat and blood vessels. The worm slithers around the body until it emerges through the upper layers of the skin. There, the worm secretes a chemical that causes a stinging blister, fever, and vomiting.

"Guinea worms do not crawl out on their own," says Ruiz-Tiben. Once a Guinea worm pokes out of the skin, it must be pulled out a few centimeters a day--a process that can take weeks to complete since the worm is so lengthy. "If one yanks the worm, and breaks it, the remaining piece of worm is left in the tissues, where it will rot," says Ruiz-Tiben. That could lead to a dangerous skin infection.

The worm-removal process is painful, Ruiz-Tiben says. But once the worm is fully removed, the patient's fever and vomiting usually cease.

Now, the Carter Center is educating villagers about filtering drinking water to keep it worm-free. This and other projects have shown success: The number of worldwide Guinea-worm infections has dropped from 3.5 million annual cases in 1986 to 10,674 in 2005.