Schistosomiasis is a parasitic infection caused by blood flukes (trematodes) of the genus Schistosoma. The intermediate host is an aquatic or amphibious snail. Humans become infected when the skin is penetrated directly by the cercaria larvae found in fresh water inhabited by infected snails. It is also known as Bilharziasis in some endemic countries after Theodore Bilharz, who described the infection in 1851.

The World Health Organization describes schistosomiasis as 'the second most important (tropical disease) in terms of public health importance' after malaria. It is endemic in 76 developing countries, putting over 600 million people at risk of infection. Over 200 million people are estimated to be infected worldwide, with 20,000 deaths annually; visitors from non-endemic countries such as the United Kingdom (UK) may also be at risk from being infected.