Schistosomiasis, or bilharzia, is one of the most widespread parasitic diseases on earth. It sickens hundreds of millions of people, most of them in sub-Saharan Africa. For years the global health world has pushed hard against it, and in plenty of places the push has worked. Infection rates fell. Communities that once carried heavy disease burdens now clear the World Health Organization’s bar for “elimination as a public health problem.” Clearing that bar is not the same as killing the parasite off. A growing pile of evidence shows that in some communities the infection keeps humming along even after the numbers look fine on paper, and then it comes back.…