A decade ago Joe DeRisi received a letter from a woman with a picture of herself wrapped in a boa constrictor. “I heard you’re a virus hunter,” the letter began before going on to explain that the snake, which she referred to as “Mr. Larry,” was her service animal. She was terrified that an illness then killing snakes all over the world might carry away Mr. Larry too. “I thought, ‘Wow, that’s crazy,’” recalls DeRisi, a biochemist at the University of California at San Francisco. It actually wasn’t much crazier than a lot of the other problems that landed on his desk. Human beings with illnesses that mystified the global medical establishment often found their way to DeRisi. Snakes, not so much. “I let the letter sit on my desk for maybe a year,” he said. “It was a weird letter.”