One cold morning in Baltimore last October, a 26-year-old named Alexander Laurenson strode into a small white room to have his arm preyed upon by mosquitoes. As requested, he had not showered the night before to make his skin more attractive to the pests, which are drawn to body odor. The mosquitoes, for their part, had been infected with malaria, a disease that kills over 600,000 people every year.

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Mr. Laurenson was part of a study to test the effectiveness of a new monoclonal antibody designed to prevent malaria transmission. Twenty-one others also arrived at the University of Maryland School of Medicine that morning, all of them part of a human challenge trial, a research method in which volunteers are knowingly infected with a pathogen.

In preparation, the room had been secured, with no cracks or seams to ensure against the mosquitoes’ escape, with a bug zapper installed on the wall and a few electrified paddles added as extra precautions. One by one, the healthy volunteers shuffled inside to serve as willing prey.

Throughout the 20th century, trials like this one have underpinned the development of vaccines for deadly scourges like typhoid and cholera. Today, they are typically undertaken only with diseases that already have fast-acting drugs to ensure recovery. But because of the perceived risk involved, and their significant cost, challenge trials remain rare and, in some cases, controversial.

“A lot of people say, ‘Doesn’t this violate the Hippocratic oath?’ or ‘How can a doctor do this?’” said Seema K. Shah, a bioethicist at Lurie Children’s Hospital and professor at Northwestern University in Illinois who studies challenge trials. “It’s just something that’s hard for the public to understand.”

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