MARY ROACH WAS never especially interested in war. She didn’t come from a military family, she never served, and she hadn’t reported on war as a journalist. But the best-selling author of books on the science of sex (Bonk), eating (Gulp), and death (Stiff) specializes in, as she puts it, “turning over rocks and writing about peculiar things.” So when Roach met with researchers in India who were using ghost peppers to make a nonlethal weapon—and discovered they were also testing leech repellent for use in the military—she sensed an opportunity. The researchers were planning to send subjects into a river during peak leech season with one bare leg and the other covered in the repellent. “I had a moment where I went, ‘Whoa. That could be a very Roach-y area,’” she says.