Former president Jimmy Carter announced Thursday that his cancer had spread to his brain. Carter, 90, will undergo radiation therapy, but his life expectancy could be less than a year.

But Carter has a selfless final wish:

It may seem like an odd request, for those lucky enough to live in ignorance of the Guinea worm. But the parasitic worm Dracunculus medinensis is no joke.

In remote parts of Africa where clean drinking water is difficult to come by, the parasite was once a common ailment. In the mid 1980s, when Carter first turned his attention to the disease, there were about 3.5 million cases annually.

Humans drink stagnant, dirty water with tiny crustaceans in it -- and those crustaceans are often host to Guinea worm larvae. Once inside a human host, the larvae will penetrate the stomach wall and live in the abdominal cavity, growing for three months before mating.

The male dies after mating and is absorbed by the host's body, so that's not so bad. But the female is more troublesome: Once fertilized, she'll travel through her hosts body underneath their skin, making her way to an extremity -- usually the foot. Then, as much as three-feet-long fully grown, she bursts through and forms a painful, burning blister.

And this is where the Guinea worm's evolutionary strategy gets creepy-good: Because of the burning sensation of an emerging Guinea worm, hosts often put their affected limbs into water to relieve the pain. That puts the fertile female exactly where she wants to be, allowing her to drop hundreds of thousands of new larvae into the water -- where a new cycle will begin.

Meanwhile, the worm's emergence means weeks of pain for the host, hurting their health and livelihood, and often causing debilitating infections.

The Carter Center's work to eradicate the parasite is credited with dropping cases from over 3.5 million when they started to just 11 known cases so far this year.

"Twenty-six thousand five hundred villages were affected — and [the Carter Center] has been to every one of them," Carter told NPR in January. Before his organization got involved, advocates were having a tough time finding anyone who wanted to focus on the disease.

"One of the first cases of Guinea worm I ever saw was a beautiful young woman who I thought was holding a baby in her right arm," Carter told NPR. "But it was actually her right breast, and it had a Guinea worm a foot long coming out of her nipple. She later had 11 other Guinea worms coming out of her body that year. On that visit we had a banker who volunteered to pay to dig a deep well for the village [so they could have clean water]. And when we went back there a year later, there was no more guinea worm. That's what happens quite often when the villages take advantage of the advice that we give them. We let them do the work, and we give them credit for it."

There's really no treatment for the parasite -- doctors still use the old method of removal, which is to slowly wind the emerging worm around a stick or piece of gauze, taking days or weeks to ease it out of the host's aching body. But by treating water to kill the parasites, and by helping communities get access to more reliable sources of fresh water, Carter and his team have come wonderfully close to eradicating the disease.

With cases dwindling, it's hard to say where and when the last will occur -- and whether Carter will still be alive to see it. But whenever the disease is officially eradicated, it will no doubt become the former president's greatest legacy.

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