A parasite that has plagued the human race since antiquity is poised to become the second human disease after smallpox to be eradicated. “We are approaching the demise of the last guinea worm who will ever live on earth,” says former U.S. president Jimmy Carter, whose Carter Center has spearheaded the eradication effort.

Unlike polio's high-profile eradication program, the mission to eliminate guinea worm disease has largely been off the public's radar. Affecting some of the poorest and most remote communities in Africa—97 percent of cases are in South Sudan—guinea worm is a parasitic infection caused by the nematode roundworm Dracunculus medinensis. It is the only disease transmitted solely by drinking water, and humans are its only reservoir, says James Hughes, professor of medicine and public health at Emory University. The disease spreads when villagers consume water containing fleas that harbor guinea worm larvae. The larvae grow to maturity inside the human body and emerge after a year as a fully grown two- to three-foot-long worm that often exits through the leg or foot. It is an excruciatingly painful process, and individuals often immerse the limb in water to cool the burning sensation, which starts the cycle all over again.

Since 1986 groups such as the Carter Center have distributed cloth water filters to villagers and educated residents about how not to spread the infection. They have also selectively used Abate, a larvicide, to control fleas in the drinking water.

So far the efforts have resulted in a 99 percent reduction in infections, says Sharon Roy of the U.S. Centers for Disease Control and Prevention. In 1986 there were 3.5 million cases, as compared with only 1,060 in 2011 and a mere five as of the first few months of 2012.