When it comes to viruses, especially the serious kind that can make you bleed from your eye sockets and wipe out entire villages, most people naturally prefer to keep their distance. Not Nathan Wolfe. The 39-year-old epidemiologist has spent the past 10 years hunting them down in the jungles of Southeast Asia and sub-Saharan Africa. By collecting thousands of blood samples from wild animals and the people who live in close contact with them, Wolfe and his team have uncovered new viruses related to HIV and smallpox. He's even documented how these animal-borne killers leap to humans, with blood serving as a vector in transmitting viruses from slaughtered animals to hunters.