Scientists have called them everything from "bad news wrapped in a protein coat" to "exquisite parasites." Either way, the invisible invaders have risen to new prominence in the age of AIDS, but, as Radetsky (coauthor, Peak Condition, 1986) reminds us, viruses have been around fora long, long time. So just chronicling the last 200 years of disease and discovery makes for a long (464-page) book, but a good one. Some of the earlier material covers familia ground: smallpox, cowpox, Jenner; Pasteur and rabies; the discovery of a "contagious living fluid," one that could pass through a filter and still infect tobacco plants. It was this agent that came to be called a virus by mm-of-the-century Dutch scientist Martinus Beijerinck. Subsequent chapters deal with the major human viruses and the diseases they cause, including influenza, hepatitis (now a family of four viruses), cold viruses (at least 200), cancer viruses, and the herpes group. The latter includes Epstein-Barr virus, the cause of mononucleosis and also associated with Burkitt's lymphoma in Africa. AIDS takes center stage toward the end of the book, leading to a discussion of how retroviruses and other viruses could be engineered to cure rather than cause disease and some speculation on their role in evolution. Radetsky mixes the science with just enough personal detail to flesh out an absorbing history, complete with past and present scandals and controversies. For an author whose previous books focused on exercise and health, and who says he knew next to nothing about viruses when he began, Radetsky sure learned fast--and well.