Many infectious diseases, like smallpox, polio, and anthrax, are ancient and have plagued humankind for thousands of years. But new strains of bacteria and viruses continue to emerge seemingly out of nowhere to cause mysterious new ailments.In Wisconsin in May 2003, three-year-old Schyan Kautzer came down with symptoms vaguely reminis cent of smallpox. Eileen Whitmarsh, a forty-two-year old pet store owner, developed flulike symptoms along with blisters on her head and under her arms. Two employees at a veterinary clinic also became sick. The one thing they all had in common was a close encounter with prairie dogs. The three-year-old had received a prairie dog for a pet, the pet store owner had had the animals in stock, and the workers at the clinic had recently treated a sick prairie dog. Blood samples taken from all the victims revealed startling news: They had a disease never...