They are medicine's rapid-reaction force, an elite cadre of highly trained disease detectives who are poised to go anywhere on a few hours' notice to investigate a threat to public health. As science journalist Maryn McKenna makes clear in Beating Back the Devil (Free Press, $26) -- her deeply researched portrait of the class of 2002 of the Epidemic Intelligence Service, part of the Centers for Disease Control and Prevention -- their work is by turns terrifying, exhilarating, gritty and exhausting. In other words, it bears almost no resemblance to network television's portrayal of medical investigators.

In the real world, as McKenna makes clear, EIS officers, many of them doctors just out of residency, do not spend their two years of on-the-job training barking orders into state-of-the-art cell phones or engaging in long, soulful chats with the relatives of deathly ill patients. Their focus is on "shoe leather epidemiology" -- conducting interviews in the field and painstakingly collecting and analyzing reams of data to ferret out the source of an epidemic.

A senior medical writer for the Atlanta Journal-Constitution who has covered the CDC for seven years, McKenna shadowed members of the first class to train after Sept. 11, 2001. Like their predecessors, many will go on to assume leading roles in state and federal health agencies or in academic medicine. But unlike previous classes in the 50-year history of the EIS, theirs was the first group whose training started with a bioterrorism drill.

One of the book's most chilling chapters involves training the group about what to do in the event of a suspected poison gas attack. During the exercise, a trainer informs a recruit whose mask wasn't sufficiently airtight, "You realize, if this were a real emergency, you'd be dead by now."

McKenna juxtaposes the history of the EIS, created in 1951 out of an erroneous concern that American soldiers had been exposed to biological agents during the Korean War, with modern examples of its mission: in Zaire, assisting overwhelmed officials with a 1994 cholera epidemic triggered by war; in Philadelphia in 2002, investigating an outbreak of listeriosis, a rare pathogen caused by tainted turkey breast.

One of the book's most interesting chapters recounts the role of an unsung EIS officer who was an integral part of a team of Los Angeles scientists who told the world about the first five American cases of a baffling sexually transmitted disease among gay men, later named AIDS.

At times McKenna gets sidetracked by minutiae, such as whether the disease detectives should be required to wear uniforms. But overall her book provides a revealing glimpse into the little-known world of deeply committed investigators who trade two years of their lives to be on the front lines of public health.

Sandra G. Boodman writes on science for The Washington Post.