Schistosomes are helminths of the class Trematoda that alternate generations, with a sexual phase in definitive mammalian hosts and an asexual phase in intermediate snail hosts. In humans, these blood flukes reside in the mesenteric and vesical venules. They have a life span of many years and daily produce large numbers of eggs, which must traverse the gut and bladder tissues on their way to the lumens of the excretory organs. Many of the eggs remain in the host tissues, inducing immunologically mediated granulomatous inflammation and fibrosis. Heavy worm burdens may produce hepatosplenic disease in schistosomiasis mansoni and japonica and urinary tract disease in schistosomiasis haematobia. Since both the schistosomes and the eggs utilize host metabolites, and since the host responses to the parasite are affected by its nutritional status, malnutrition may strongly affect both the parasite and the complex host-parasite relationship.