Schistosomiasis is also called bilharzia. It is a chronic conditions caused by small flatworms (flukes) that live inside blood vessels where they lay hundreds of eggs a day. The fluke's eggs are eventually passed out in faeces or urine. They hatch in fresh water and then enter the right kind of snail. Inside the snail they develop to a further stage which is released back into the water and that subsequently can enter a person through the skin. The initial illness is due to an allergic reaction to these parasites. Within 1 to 3 months they develop into adult flukes inside the liver which then find places in the body to lay their eggs. The eggs can damage the liver or bladder and sometimes other organs. The adult flukes can live for 20 years. Treatment to kill them is usually effective if started early.
Prevention is by improved hygiene and not swimming or bathing in water where there are infected snails.
Parasite is a term for all organisms larger than bacteria and viruses that can cause disease by living inside a person, animal or plant. After malaria, Schistosomiasis is probably the most serious worldwide human parasite infection, affecting about 200 million people.
Schistosomiasis caused 0.08% of all deaths worldwide in 2002, with an average of 7 deaths per million people. But that is only death directly from schistosomiasis, and does not include estimates of deaths that may be due to schistosomiasis from subsequent bladder cancer, cirrhosis of the liver, or colon cancer.