The other month, in South Sudan, I sat with a nine-year-old named Nakal twice a day. She had come from the Mogos containment-care center, in Eastern Equatoria, where, for two weeks, she was tortured by a guinea worm emerging from her right knee. It infiltrated her body last year through a single gulp of unfiltered water she took on a scorching day while tending crops with her grandmother. Now, it was coming out—slowly. One day, I watched as the local medical staff managed to coax four inches of the reluctant worm from behind Nakal’s swollen knee. The animal, which looks a bit like a wet, sticky noodle, dangled out. As she winced, Nakal allowed only a single tear to fall down her cheek.

The life cycle of a guinea worm, Dracunculus medinensis, is deviously simple. Cool, clear water teems with life, much of it microscopic crustaceans; this is normal, and typically without consequence. But sometimes, resident water fleas host the larva of a rapacious, fiery serpent. When ingested, the water-flea hosts dissolve in stomach acid, allowing the larval worms to move through the human body, where they mature and mate. As the females of the next generation grow, they pack their own bodies with live young. During this period, the victim suffers no immediate symptoms. A year later, the three-foot-long mother awakens deep in her host’s leg. She rises to the surface and induces a blister on her victim’s skin that is so fierce, and of such searing agony, that the afflicted seek relief in the coolness of water. Here, the mother dumps her offspring, completing the life cycle for another generation. The guinea worm has repeated this process for hundreds of thousands of years.

The guinea worm can only reproduce at the expense of those it infects. Adult worms only infect humans, while the larvae only invade water fleas. It is also the only human parasite that must cause pain in order to successfully reproduce. In addition to deeply wounding and torturing its victims for weeks on end, dracunculiasis is a disease of starvation. It leaves families in affected areas incapacitated and unable to reap the crops they have sown.

There is no vaccine for guinea worm, and there are no drugs that can cure those who are infected. The pest once afflicted hundreds of millions of people from the Gambia to India. But the worm is now gone from Guinea, and from almost everywhere else. At last count, there were only five hundred and forty-two people infected, down from an estimated 3.5 million in 1986. Of the remaining cases, exactly five hundred and twenty-one are in South Sudan.

We know these numbers with precision because of a campaign that President Jimmy Carter began in 1986 to destroy the worm. That community-driven process, coördinated by the Carter Center and executed by the South Sudan Ministry of Health, village volunteers, and trained technical advisors, is driving the parasite out of its last remaining human hosts. Each person known to be infected with the worm, along with every village in which outbreaks have occurred or are occurring, is tracked. Intervention is multi-pronged: the infected are quarantined, since patients with emergent worms must be kept away from water sources; containment-care facilities welcome not just the victims but their dependents, too; local education programs focus on water filtration.

Many years ago, when I was beginning a career in parasitology, I was told about the “parasitologists dilemma,” a spin on the Malthusian catastrophe. The premise was that, should parasitologists actually manage to eradicate infectious diseases caused by parasites, we’d be morally responsible for thrusting millions of people into the inevitable conflict and poverty that would result from population growth and increased pressures on a limited food supply. But in every place where the guinea worm has been eliminated, the very opposite is true. Just a couple of decades ago, Ghana and Mauritania had thousands upon thousands of cases. Now, both countries are free not only of guinea worm but, increasingly, of the shackles of poverty: per capita G.D.P. has tripled, the number of people living on less than a dollar a day has been halved, and the birth rate has plummeted. This, obviously, is the result of many factors besides guinea worms. But I can say this: so much for Malthus.

My last day in South Sudan was the hardest. A forearm’s length of the worm was draped on Nakal’s thigh. More of it was gently pulled from the deep tissue behind the back of her knee. As it tore through her flesh like a skein of dry bamboo, each meagre inch was more agonizing than the last. Nakal, relenting, finally let her tears flow.

Most conservation efforts, like those for spotted owls and pandas, are proxies for threatened ecosystems, or seek to preserve “keystone species,” like sharks. Few focus on the despised and downtrodden species that crawl beneath our feet. As a leech expert, “save the maggots” has always been my rallying cry. The rains have come to East Africa again, and with them come the worms. Let no one mourn this extinction.

Dr. Mark Siddall is the curator of Division of Invertebrate Zoology at the American Museum of Natural History and a professor at the Richard Gilder Graduate School.

Photograph by Mark Siddall/AMNH