fossils from the Devonian of Latvia, at least some of which might represent parasitic monogeneans. Interaction with humans Parasitism Cestodes (tapeworms) and digeneans (flukes) cause diseases in humans and their livestock, whilst monogeneans can cause serious losses of stocks in fish farms. Schistosomiasis, also known as bilharzia or snail fever, is the second-most devastating parasitic disease in tropical countries, behind malaria. The Carter Center estimated 200 million people in 74 countries are infected with the disease, and half the victims live in Africa. The condition has a low mortality rate, but usually presents as a chronic illness that can damage internal organs. It can impair the growth and cognitive development of children, increasing the risk of bladder cancer in adults. The disease is caused by several flukes of the genus Schistosoma, which can bore through human skin; those most at risk use infected bodies of water for recreation or laundry. In 2000, an estimated 45 million people were infected with the beef tapeworm Taenia saginata and 3 million with the pork tapeworm Taenia solium. Infection of the digestive system by adult tapeworms causes abdominal symptoms that, whilst unpleasant, are seldom disabling or life-threatening. However, neurocysticercosis resulting from penetration of T. solium larvae into the central nervous system is the major cause of acquired epilepsy worldwide. In 2000, about 39 million people were infected with trematodes (flukes) that naturally parasitize fish and crustaceans, but can pass to humans who eat raw or lightly cooked seafood. Infection of humans by the broad fish tapeworm Diphyllobothrium latum occasionally causes vitamin B12 deficiency and, in severe cases, megaloblastic anemia. The threat to humans in developed countries is rising as a result of social trends: the increase in organic farming, which uses manure and sewage sludge rather than artificial fertilizers, spreads parasites both directly and via the droppings of seagulls which feed on manure and sludge; the increasing popularity of raw or lightly cooked foods; imports of meat, seafood and salad vegetables from high-risk areas; and, as an underlying cause, reduced awareness of parasites compared with other public health issues such as pollution. In less-developed countries, inadequate sanitation and the use of human feces (night soil) as fertilizer or to enrich fish farm ponds continues to spread parasitic platyhelminths, whilst poorly designed water-supply and irrigation projects have provided additional channels for their spread. People in these countries usually cannot afford the cost of fuel required