Germ theory was never a given. This now-commonplace idea — the notion that human diseases can be sparked by tiny pathogens infiltrating the body — emerged on the backs of discoveries made by people over time. Those discoveries steadily slotted together to form a bigger picture, revealing both the wonders and terrors of the microbial world around us.
Thomas Levenson, a professor of science writing at MIT and author, traces the history of germ theory from its inception to the present day in a new book called "So Very Small: How Humans Discovered the Microcosmos, Defeated Germs — and May Still Lose the War Against Infectious Disease" (Random House, 2025). In the book, Levenson also tackles the larger question of how and why new ideas are pursued, accepted or ignored.
