BookBrowse Reviews So Very Small by Thomas Levenson

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An engrossing history of one of the most underrated breakthroughs in science.

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Not much about the COVID-19 pandemic could be called "lucky," but the fact humanity faced it in the 21^st century—with all modern medicine's tools at our disposal—has made a remarkable difference in how we've been able to respond. Within a few weeks of the virus's emergence, scientists had already mapped its genome; within a few months, the first vaccines were in trials. And within a few short years (however long they may have felt), humans had reined in a global pandemic that had threatened to overwhelm health systems and claim the lives of millions more. Despite the undeniable losses the virus caused and continues to cause, what happened was nothing short of miraculous: the danger abated, societies reopened, and many of us now take for granted the incredible scientific achievements that allow for the current state of things.

Science writer Thomas Levenson, however, is not one to take such things for granted; even a hundred years earlier the story of COVID would have been entirely different. With So Very Small, his latest in a string of popular histories of science, Levenson attempts to educate the rest of us in how good we have things today. The book centers on the history of germ theory, tracing a centuries-long struggle to understand the microscopic lifeforms (bacteria and viruses) that were for so long the unknown root of so much of humanity's suffering.

Levenson begins his tale in London in 1665, during the last major outbreak of bubonic plague before the discovery of microbes, and he ends it in the present day, after the COVID-19 pandemic's acute phase. That is to say, he takes his reader from a time when people believed epidemics to be caused by "miasmas" (some ill-defined putrescence carried on the air) to a time when the medical world bases itself on germ theory: a powerful understanding of infection as a "straightforward sequence of cause and effect"—with the logical corollary being that this chain can be broken. It's a revolution in medicine that Levenson not unreasonably calls "as great a breakthrough as any in the history of science."

So Very Small approaches the history of germ theory through the figures who marked the road to discovery; some achieved fame outside scientific circles (Louis Pasteur, Alexander Fleming), while others who offered equally important contributions are less appreciated by the English-speaking public (Antoine van Leeuwenhoek, Filippo Pacini). What's perhaps more interesting than these case studies, though, is Levenson's central thesis about why it took so long—two hundred years, almost to the day—to go from discovering microorganisms to implicating them in disease processes. The main culprit, he argues, was hubris: the belief that humans sat atop the natural world, only God above them, and therefore couldn't possibly be undone by something as small and insignificant as a germ. It's as much a point about centuries-old arrogance in the face of public health crises as it is about that very same problem today.

For the most part, though, Levenson avoids throwing himself into contemporary debates. His focus is the story and, author of half a dozen history books, he is an experienced and gifted storyteller. He may hold the title of Professor of Science Writing at MIT, but he comes across more like an eager high school teacher with an infectious enthusiasm for his chosen subject. His narration—dotted with exclamation points and "can-you-believe-that?" asides to his reader—ensures the story clips along at a reasonable place, while still reveling in moments of grisly melodrama. At times that enthusiasm can veer him off track, leading him toward topics whose relevance might be a stretch (the history of certain military ordnance, say, or Darwin's theory of evolution). But these lapses in focus are forgivable; more than anything, they seem to speak to Levenson's irrepressible joy in sharing knowledge.

The book's first half, detailing the scientific landscape in the centuries before any understanding of microbiology, is by far its more engaging. Levenson captures the drama of great thinkers groping in the dark, blind to what he dubs the "microcosmos" and blinded by assumptions about humanity's place in a divinely ordered world. The second half of the book, touching on discoveries in various disease areas after germ theory has been elaborated, proves that what's good for humanity isn't necessarily good for Levenson's narrative. The mystery has evaporated, and with it the stakes: the recounting of breakthrough after breakthrough in chapter after chapter can't help but feel somewhat repetitive.

Still, there is more than enough to enjoy in this engrossing account of what even today could be considered a silent revolution. Within living memory, Levenson writes, humans eliminated "the terror of disease and injury as the constant undercurrent to daily life." It's a change so embedded in our worldview as to be unnoticeable. So Very Small is a celebration of the centuries of accumulated knowledge that got us there—and a forceful warning not to squander it.

This review first ran in the June 4, 2025 issue of BookBrowse Recommends.