The Guinea Worm is a parasitic nematode which inhabits areas from West Africa to India. Infectious larvae develop inside of aquatic copepods in untreated water. When consumed, the little critters develop and mate inside of the host's intestinal tract. After mating, the male dies and the female (which can grow up to 3 meters in length) burrows out of the gut and under the host's skin to one of the lower limbs (between the knee and ankle generally), forms a blister, pokes her head out of the blister, and releases up to 3 million eggs next time the host enters the water. The female dies after laying eggs and the body's immune response to the dead worm body under the skin can cripple the host. It turns out that the first recorded medical prescription in history was for dealing with an emerged female Guinea worm. The ancient treatment (which is remarkably similar to the modern solution) involves slowly winding the worm's body around a stick (picture wrapping thread around a spool, except the thread, which is inside your leg, is a 10 foot long parasitic worm full of hundreds of thousands of baby parasitic worms who's body can cause major chronic pathology if you can't get it all the way out. No pressure.). Here's a paper I found which discusses the measures people are taking to eradicate this disease world-wide.