Germ theory is the idea that pathogens can invade the human body and cause disease — and it wasn't always accepted. Evidence for germ theory accumulated over time, and as it did, it butted against existing explanations of how and why illnesses manifest. Yet now the theory is central to our understanding of why many diseases occur, as well as to how they can be prevented and cured.
In a new book, Thomas Levenson, a professor of science writing at MIT, traces the history of germ theory while tackling the broader question of why some ideas take hold and become accepted while others are ignored. The book — called "So Very Small: How Humans Discovered the Microcosmos, Defeated Germs — and May Still Lose the War Against Infectious Disease" (Random House, 2025) — brings the reader all the way to the present day, as humanity's struggles with germs continue in the form of antibiotic resistance and a new flavor of anti-vaccine sentiment.
