No disease is as clearly linked with development as schistosomiasis. Nicknamed the “germ of progress” in the 1970s for its close association with water development projects that allowed freshwater snails to thrive, schistosomiasis remains the “silent scourge of development.” Though preventable and treatable, it flourishes throughout the developing world. It has infected 200 million people worldwide, and threatens an additional 400 million, primarily in Africa, Asia and Latin America. Of those infected, 120 million people are symptomatic, 20 million have severe complications and 200,000 die each year, more than the number of deaths from AIDS, diabetes, kidney disease, pneumonia and influenza combined in the United States.

Humans are at risk when they enter freshwater sources infested with snails that release fork-tailed parasites called cercariae. Cercariae burrow through skin and grow into worms that inhabit the capillaries of the intestines or bladder. The worms mate and lay barb-tailed eggs that cut through vessel walls, eventually leaving the body in the feces or urine. When people relieve themselves near snail-infested water, the eggs enter the water and hatch, then grow into parasites called miracidia that lodge in the shells of specific snail hosts. Each miracidium gives rise to thousands of cercariae, and the cycle explodes.

An initial acute infection with schistosomiasis is followed by chronic debilitating disease. After an initial itchy rash at the site of cercaria entrance, accompanied by fever, weeks go by with minimal symptoms. The cercariae grow into worms that use molecular mimicry to evade detection by the immune system. Schistosome eggs, not the worms, bring disease. Anemia—often severe, sometimes deadly—accompanies blood loss in diarrhea or urine. In many regions with endemic urinary schistosomiasis, a boy is considered to be a man after blood appears in his urine, thought to be a form of male menstruation.

Severe complications ensue when excess eggs wash deeper into the body, eliciting an inflammatory immune response. Liver scarring clogs the return of blood to the heart. Initial heavy infection with intestinal schistosomiasis can kill with bloody diarrhea, or cause irreversible paralysis if eggs are washed into the spinal cord.

Communitywide access to clean water, sanitation and snail eradication programs can prevent schistosomiasis outbreaks. Praziquantel, a miracle drug discovered in the 1970s, kills both the worms and their eggs in a single dose with minimal side effects. It does not prevent reinfection, but it does prevent serious complications of heavy infection. Unfortunately, all these measures to prevent and treat schistosomiasis are costly, often beyond the means of developing communities. (Happily, the cost of praziquantel recently dropped considerably, increasing access worldwide.)

Schistosomiasis is a disease of poor sanitation, contaminated water, ineffective health systems and unaffordable medications. It threatens impoverished communities struggling to build better lives.