Seventeen volunteers in the Netherlands have agreed to host parasitic worms in their bodies for 12 weeks in order to help advance research toward a vaccine for schistosomiasis, a chronic disease that afflicts more than 200 million people a year, killing thousands, primarily in sub-Saharan Africa and South America.

“Yes it sounds odd and crazy. The idea of having a worm grow inside you is awful,” says Meta Roestenberg, an infectious disease physician at Leiden University Medical Center, who is directing the research. But she said the risk to the student volunteers is “extremely small,” especially compared with the potential benefit to preventing a disease that burdens millions of the world’s poorest people. A Dutch ethics board agreed.

But other researchers of this disease are conflicted about the study’s method, fearing there is no way to be sure that all of the tiny parasites have been evicted from the hosts when the trial ends.

Schistosomiasis is sometimes called bilharzia or snail fever because the illness-causing larvae spend their nights tucked away in the shells of snails in freshwater lakes. During daylight, thousands of Schistosoma mansoni head out across the water, penetrating the skin of bathers or fishermen. Over the coming weeks, larvae turn into adult worms and males and females pair up and mate, producing hundreds of eggs a day.