This detailed account of the role of microbes in human history offers an important corrective to the widespread belief that exercise, hygiene, and the right diet are all you need to live a long and healthy life. Karlen (Threesomes, 1988, etc.) begins by pointing to the emergence in recent years of a host of new diseases: not just AIDS and Lyme disease, but such terrible (though as yet rare) scourges as the Ebola and Marburg hemorrhagic fevers and hantaviruses. In addition, the resurgence of some ancient enemies, including plague and tuberculosis, has made headlines in recent years. Karlen sees these outbreaks as new skirmishes in a war that never really ended, despite the triumphs of Louis Pasteur, Robert Koch, and other 19th-century pioneers of the germ theory of disease. Humans and microbes coexist in an ecology, notes the author, and both constantly adapt to environmental pressures. Humans deploy a range of defenses, from sickle-cell trait (against malaria) to inoculation, while microbes mutate, as the flu does every season, or adapt to new hosts, as AIDS appears to have done in the past few decades. In some cases, human penetration into new environments has exposed our species to diseases against which it has no defenses; Ebola fever may be the most recent example. In others, our overenthusiastic use of antibiotics has bred drug-resistant strains, notably of the TB bacteria. Karlen could profitably have provided more statistical detail, especially in light of the introduction's apparently hyperbolic forecasts of deadly worldwide pandemics, but on the whole this is a well-written and thought-provoking survey of an important subject. An excellent starting point for fuller study, thanks to a well-annotated reading list that includes popular texts as well as primary sources.