Legend has it that vampires create no shadows, cast no reflection and—in more modern versions of the tale—cannot be captured on photographs, film or video. Of course, vampires are only myths. Unfortunately, schistosomes, which behave in some similar ways, are not. These infectious worms dwell in human veins and eat our blood. Among parasitic illnesses, the World Health Organization ranks schistosomiasis, the disease caused by the worms, second only to malaria in terms of the number of people it kills and chronically disables and the drag it imposes on the social and economic development of nations. And, in their own way, schistosomes have achieved invisibility. Cameras can capture these creatures, but our immune system does not.