The discovery and confirmation of the existence of germs in early modern times would not have been possible without the invention of the microscope. It is true that shortly before this instrument appeared a few perceptive individuals made some educated guesses that tiny living things might have something to do with causing disease. During the mid-1500s, for example, as microbiologists Barry E. Zimmerman and David J. Zimmerman point out:

An Italian physician, Gerolomo Frascatoro, suggested that [the serious disease] syphilis was transmitted sexually by a contagium vivum, or "living agent." He outlined the different modes through which these living agents, and those of other diseases as well, could be spread: direct contact with an infected person, handling of contaminated materials, and breathing in infected air. About two hundred years later, in the mid-1700s, the Austrian physician Marcus von...