The major types of worms that cause schistosomiasis in humans are [Schistosoma mansoni], [S. haematobium], and [S. japonicum]. Certain freshwater snails are the intermediate host for the worms. The infected snails release large numbers of very small, free-swimming larvae which are capable of penetrating the unbroken skin of a human host. They may pass through the heart, lungs, and liver before ending up as adult worms in the veins of the intestines. There they lay vast numbers of eggs that work their way into the intestinal tract and are eventually passed out in the faeces. The body's reaction to the eggs produced by these worms, and not the worms themselves, causes the symptoms of schistosomiasis.
The number of flukes acquired by an individual is related to the number of water contact episodes and to the degree of body exposure. People with the worm infections develop ballooning stomachs caused by water retention, along with large livers and spleens. The infection causes a variety of intestinal and liver complications which weaken the sufferer and eventually may cause severe disability and death. Although the direct mortality from infection is low, the importance of the disease lies in the sheer size of the epidemiological phenomenon. As the human population grows, the problem intensifies on two counts - increased excretal pollution of water, and the extended use of irrigation land for crop-growing - while the creation of man-made lakes has encouraged the spread of the disease. Disease conditions are maintained by poor agricultural practices and unsanitary habits. In many areas of the world, people defaecate in rivers and irrigation canals.
Travelers are at risk when going to an area with reported incidence of the infection and when wading or swimming in lakes or rivers in rural areas where poor sanitation and the appropriate snails are present. 90% of the cases contracted by British tourists can be traced to Malawi.