The latest in Asimov's lightweight science history series covers the well-worn path from Leeuwenhoek's observations with his primitive lenses to the discovery of the tobacco mosaic virus. The gradual disproving of the theory of spontaneous generation and the contributions to germ theory of Jenner, Pasteur, Lister, Koch and Ehrlich are surveyed, but without adding either data or clarity to existing material at this level. And — a minor point but indicative of the level of care and precision — the explanation of Behring's work on antitoxins could give the impression that all disease bacteria operate via toxins. Marginal.