Trematode parasites in the phylum Nematoda include the internationally troublesome schistosomes that account for infections in about 200 million people in 75 tropical and sub-tropical countries. Of the three main species pathogenic to people, the human blood fluke (Schistosoma mansoni) has recently captured the imaginations of geneticists involved in the World Health Organization-sponsored Schistosome Genome Project. The goals of this ambitious project are to construct the physical map of this trematode's genome with hopes of discovering novel chemotherapeutic targets and vaccine candidates.

Sex-specific gene expression in S. mansoni is of interest to some genetic parasitologists. Apparently, males can induce gene expression in females. On the molecular level, signal-transducing molecules transfer the male inductive signal to proliferating germ cells in the female's reproductive organs. Male-female pairings of Schistosoma parasites lead to the induction of female fertility and egg formation.

Among human parasitic diseases, schistosomiasis ranks only second to malaria in terms of worldwide public health risks and socio-economic impacts. Known also as bilharziasis, the disease was recognized since the time of the Egyptian pharaohs, but not isolated as a parasitic helminth until 1851 by young German pathologist Theodor Bilharz. Acute infections, mental retardation, and even a type of bladder cancer are attributed to these intestinal trematodes. Eggs travel to and develop in the liver, bladder, or intestine and rarely to the brain or spinal cord, where they mature to cause seizures, paralysis or meningitis.

Like the other forms of schistosomiasis, the blood flukes are carried by dirty water, placing farmers and fishermen at a particularly high risk. In addition to pharmaceutical treatment of infected people, S. mansoni control through improvements in sanitation and water treatment, as well as snail control, is feasible. Apparently, water resource development projects, such as damming rivers, increase the prevalence of the disease as artificial shallow snail habitat is created from flowing waters.

The key to the life cycle and control of many forms of schistosomiasis are aquatic snails. Freshwater ecosystems become infested with eggs when infected humans urinate or defecate in the water or human wastes are applied as fertilizers to crops. As the eggs hatch, if certain snails are present, the parasites enter and grow. The parasite leaves the intermediate host and enters the water where it survives for approximately 48 hours, unless a human host is found. Blood flukes penetrate the exposed skin of humans that are wading, swimming, or washing with contaminated water. Within several weeks, the parasitic nematodes grow inside the blood vessels of the host and females produce eggs. Some of the eggs make it to the bladder or the large intestines, where they are then passed in the urine or stool.