Scattered through the galaxy are thousands of worlds colonized by humans. Many have native microbes dangerous to the human immigrants. Others have diseases brought to them accidentally—or on purpose—by visiting ships. When millions of lives are threatened, it's a job for the Interstellar Medical Service, and a Med Ship is sent to solve the problem. Calhoun is the best the Med Service has, and hard experience has taught him that often the major obstacle to curing the sick is ... the sick. And removing that kind of obstacle may take very strong medicine. To find a cure for a disease, Calhoun has the help of his small animal companion Murgatroyd, a formal—a species with the most powerful immune system in the galaxy. But to find a cure for hysteria, prejudice, crime, and even war is much more complicated, requiring considerable ingenuity. Fortunately, ingenuity is something that Calhoun has in good supply... In this story Calhoon finds a planet where all the people have fled and he must find out why. - Summary by Wikipedia and Phil chenevert