Schistosomiasis

Schistosomiasis is a chronic parasitic disease endemic to freshwater areas of Asia, Africa and South America. Although it rarely causes death, it can result in cognitive development issues in children and puts adults at a higher risk of bladder cancer. The disease affects up to 200 million people and only malaria has greater monetary impact on the developing world.
The disease is caused by several species of fluke worms, each of which has an identical life cycle involving humans and fresh water snails. The parasite's eggs are found in the feces of infected humans. Once the feces reach water, the eggs hatch into free swimming worms which can burrow into the foot of a snail. The parasite uses the snail as a host to produce thousands of larvae. Those larvae are released back into the water, where they can burrow through the skin of humans that swim in the water. From there, the larvae move to the lungs and then to the liver. The fluke lives there off of red blood cells and produces thousands of eggs which are released into the bloodstream where they are small enough to pass though the wall of the intestines to be excreted in feces to start the life-cycle again.
Once infected, a person generally plays host to the parasite for their entire life.
The disease is usually diagnosed by finding the eggs in stool as the worm is only about 10mm long and is rarely found in the liver except post-mortem.
Modernization ironically allowed the disease to spread as dams and irrigation usually provided the snails with an excellent habitat. Although there is no vaccine for the parasite, the focus is on prevention by eliminating or treating the habitat of the snails that carry the disease. Treatment of the disease is an annual dose of praziquantel, which is also used as a preventative when the disease is common in a population.
The eggs of the parasite that do not reach the intestine can provoke a severe immune response. This immune response, rather than the eggs itself, are what usually appear as the symptoms of the disease. Cerebellar schistosomiasis delayed hypersensitivy allergy is a rare condition caused by a severe allergic reaction to the eggs or pieces of the eggs in the body.
Schistosomiasis at Wikipedia