Bilharzia. Gross specimen of a lung showing damage (grey, right) caused by bilharzia (schistosomiasis), a disease caused by parasitic worms. The organisms (a type of fluke) inhabit tropical lakes and rivers, completing part of their life cycle in snails. They leave the snails as cercariae, which can infect humans by burrowing through the skin as they bathe. The flukes attach themselves in the veins of the intestine and bladder, and release eggs into these organs. The eggs are then expelled with urine and faeces, and pass back into the water. Chronic infections can damage many organs, including the lungs (as here), brain and liver. Drugs can treat the disease.