Chagas can cause anything from mild headaches to eventual heart failure. The condition is usually spread by what's called the kissing bug—a relative of bedbugs, which also feeds on blood. But it doesn't pass the parasite through its saliva, as happens with malaria, for example. "You get it when the bug bites you and then defecates on you. The parasite is in the feces of the bug." Michael Levy, a disease ecologist at the University of Pennsylvania. Levy says scratching the bite then works the parasite into the skin, infecting the victim. Levy and his colleagues wanted to see if the same thing could happen with bed bugs. So they let em loose on mice that carried the Tripanosoma cruzi parasite. The bugs caught it—and were then able to transmit the parasite back to mice. With one caveat—some mice actually hunted and ate the bugs—a sure route for infection.