Schistosomiasis, also known as bilharzia, is a disease caused by parasitic worms that require two hosts: humans and certain species of snails. There are two forms of the disease, namely, intestinal schistosomiasis, caused by Schistosoma mansoni and S. japonicum, and urogenital schistosomiasis, caused by S. haematobium. There are less common schistosome species in some parts of the world, e.g. S. mekongi and S. intercalatum. Schistosomiasis ranks second only to malaria as the most common parasitic disease worldwide.