“Can you identify this worm found in my toilet bowl after peeing?” asks this reader in her submission regarding the red, semi-transparent worm pictured below. “Doctor I visited was very uncertain but suspected it was a schistosoma.”

Blood flukes, which make up the genus Schistosoma, are a type of parasitic flatworm that cause schistosomiasis, an infection that afflicts tens of millions of people a year. Schistosomiasis is in fact the second most socioeconomically harmful parasitic disease in the world, according to the World Health Organization. (Only malaria is more harmful.) Below we provide a brief guide and overview of blood flukes that includes the most essential information about this parasitic disease. We will also provide links to some of the best sources for information about blood flukes online.