We treat disease as our enemy, and germs and infections as things we battle. But what if we've been giving them a bum rap? In this witty, engaging book, evolutionary biologist Zuk makes us rethink our instincts as she argues that disease is our partner, not our foe.From the earliest days of life on earth - parasites spurred the creation of complex life forms - disease has evolved alongside us, becoming not only natural but essential to our health. Drawing on the latest research and most unusual studies, Zuk explains the role of disease in answering a fascinating range of questions: Why do men die younger than women? Why are we attracted to our mates? Why does your average male bird not have a penis? Why do we - as well as insects, birds, pigs, cows, goats, and even plants - get STDs? Why do vultures have yellow heads and roosters have red wattles? Why do we have sex at all, rather than simply splitting off copies of ourselves like certain geckoes? And how is our obsession with cleanliness making us sicker?