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Before germ theory, disease seemed unknowable—a punishment, a mystery, a fate. After it, we could fight back. But the leap from the first glimpse of a microbe to the realization that these tiny beings could kill us took 200 years.
We often tell the story of germ theory as a straightforward triumph: microscopes, microbes, medicine. But discovery isn’t the same as acceptance. When I started asking what held us back, the answer wasn’t about science alone—it was about psychology, belief, and power.
The delay wasn’t just a matter of missing evidence. It was about how the Western tradition understood the place of humankind in the hierarchy of nature. For a very long time, and for almost everyone, the possibility that microbes might be pathogens—specific agents of disease—was almost unthinkable. How could such small and simple creatures have an impact on human beings, endowed by their creator or their big brains with dominion over the natural world?
That’s when I realized the story of germ theory wasn’t only about microbes. It was about us.
In Aberdeen, Scotland, in 1795, a physician named Alexander Gordon noticed something strange: The deadly malady puerperal fever, which killed new mothers, seemed to follow him around. It moved with the people attending births—midwives. And him.
He didn’t know what he and his colleagues were carrying with them to each delivery. But he knew they brought something.
We imagined invisible enemies long before we caught them under a lens.
That insight—specific, careful, ignored—was rediscovered again and again over the next half century. Doctors suspected cholera was transmitted by water. They noticed that outbreaks of puerperal fever moved through hospitals in waves. The idea was there: Specific diseases might have particular physical sources. They could be passed body to body, hand to hand.
But they had no names for the culprits. No way to see them.
As I researched and wrote my book, I began to realize how essential this hazy middle ground was—between intuition and confirmation, between dread and proof. We imagined invisible enemies long before we caught them under a lens. And every year we failed to connect the dots, more people died.
When I started this book, I had the timeline wrong. I knew the polio vaccine arrived in the 1950s. But as I remembered it, the rest—all the childhood vaccines we now take for granted—were already in place. When my son was born, he got the whole catalogue of shots. Not only did he escape bouts with measles, whooping cough, mumps, and the others, it never occurred to me that he might not—and I forgot how different my own childhood had been in the late 1950s and early ’60s.
Rubella, the final standard childhood vaccine to be added to the slate, didn’t arrive until 1969—too late for me. My terrible week with Rubella all came back to me when I started to research our post-World War II victory over infectious disease, which we won largely due to the development of our growing catalogue of vaccines.
The disease found me in the mid-60s, when I was just a kid. I suffered a fever, a rash, and several days of its characteristic discomfort—which means I was one of the lucky ones. So many others who got it as mothers or infants faced lasting consequences: severe birth or developmental defects, miscarriage, the death of the fetus or child.
I’m hardly alone in my amnesia. It’s been just over half a century since parents in the United States worried daily about measles, polio, mumps, and diphtheria. Those illnesses—once common, once terrifying—had, until recently, faded from view. But now, a measles outbreak that began in Texas has killed at least two unvaccinated children and has spread to more than 20 states. Diseases we once thought were behind us are back.
Of all the insights I had while working on this book, this one cut deepest: Humans are excellent at forgetting how to be afraid when a particular danger is not right in their faces—even when their forebears fought so hard for that freedom. 
Lead image: roroiishaa / Shutterstock
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