An itchy, painful blister was the first sign that this child from a rural village in Ghana, Africa, had been infected. Then the sore broke open. Inside was a thin white Guinea worm. Health worker Abaare Hussein had to gently pull the1 meter (3 foot)-long parasite from the child’s leg bit by bit—a process that can take months.

Guinea worms start life as immature larvae inside water fleas. When villagers drink unfiltered water, they also gulp down these tiny crustaceans. The larvae get released inside a person’s digestive tract, where they grow into adults. After a female worm mates, it travels to the skin’s surface, usually on a person’s lower limb. There, the worm releases fluid that produces a painful, searing blister. To seek relief, people often dunk the burning area in water. That triggers the worm to burst from the wound, along with tens of thousands of new larvae.

In the 1980s, there were about 3 million annual cases of Guinea worm infections in 21 countries across Africa and Asia. Today, that number has plummeted to about 30 a year. That’s thanks to a global effort to eradicate the parasite. The campaign aims to bring the number of cases to zero by improving access to safe drinking water and educating people about how to avoid the parasite. Local communities were quick to adopt these efforts. “Once someone has had a Guinea worm, they never want one again,” says Adam Weiss, director of the Guinea Worm Eradication Program at the Carter Center in Atlanta, Georgia.