Among these diseases, the devils we know are bad enough. Mr. Quammen also thinks determinedly about what he calls the NBO’s — the Next Big Ones. “Will the Next Big One come out of a rain forest or a market in southern China?” he asks. “Will the Next Big One kill” 30 million or 40 million people? He makes you dread that sneeze at the back of the bus.

Mr. Quammen, whose previous books include “The Song of the Dodo” (1996) and “Monster of God” (2003), is not just among our best science writers but among our best writers, period. (Check out his much anthologized short story “Walking Out,” about a father and son gone hunting, if you want a taste of his fiction.) That he hasn’t won a nonfiction National Book Award or Pulitzer Prize is an embarrassment.

“Spillover” is a work of synthesis, not original science, and Mr. Quammen is generous about crediting his sources. But he doesn’t shy from serious science, to the extent that he sometimes apologizes for going overboard:

“If you followed all that, at a quick reading,” he writes on Page 136, “you have a future in biology.” (I suffered flashbacks to my least favorite college courses.) His zest for honest science leads him into one of his book’s most interesting detours, a polite but rigorous takedown of Richard Preston’s 1994 best seller “The Hot Zone,” about an outbreak of the Ebola filovirus.

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Mr. Preston’s book filled our heads with “wildly gruesome notions” about how Ebola patients die, Mr. Quammen suggests, and he shoots down many of what he and others feel were Mr. Preston’s theatrical exaggerations.

“They don’t explode, and they don’t melt,” one expert from the Centers for Disease Control and Prevention tells the author about Ebola patients, in exasperation with “The Hot Zone.” Mr. Quammen declares that the Ebola virus generally kills “with a whimper, not with a bang or a splash.”

For those of us who don’t have a future in biology, Mr. Quammen is a patient explainer and a winning observer. His gallows humor is superb. “Advisory: If your husband catches an ebolavirus,” he says, “give him food and water and love and maybe prayers but keep your distance, wait patiently, hope for the best — and, if he dies, don’t clean out his bowels by hand. Better to step back, blow a kiss and burn the hut.”

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He has a novelistic flair for describing his fellow humans. A Swiss microbiologist has “a shovel-wide jaw, a cagey smile” and “a great domed head like Niels Bohr.” A British theoretician of viral evolution wears “wiry glasses with a thick metal brow, as in old pictures of Yuri Andropov.” He’s not bad on animals either. A type of monkey called a sooty mangabey resembles “an elderly chimney sweep of dapper tonsorial habits.”

His section on Lyme disease may change what you think you know about the illness. Deer are almost irrelevant to the spread of Lyme disease, Mr. Quammen contends. Smaller mammals — their natural predators like foxes, owls and hawks less prevalent — are what put us at risk. Having said that, I cannot shake his disquieting observation that “One poor doe might be carrying a thousand mature black-legged ticks.” Fie on that mental image.

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Reviewing the film “Get Rich or Die Tryin’ ” in The Sunday Telegraph in 2006, the novelist Zadie Smith said about the performance of its lead actor, the rapper 50 Cent: “My brain is giving you one star, but my heart wants to give you five.” My feelings about “Spillover” are the inverse. My brain declares five stars. My heart, however, awards it 3.5. The book feels haphazardly organized. It drifts into eddies. Its Tolstoyan length includes padding. It is a very long river to float.

It is worth persevering. Mr. Quammen, combining physical and intellectual adventure, wraps his canny explorations into powerful moral witness. Our disruption of the natural world, “Spillover” declares, is largely to blame for unloosing terrible microbes.

“When the trees fall, and the native animals are slaughtered,” he writes, “the native germs fly like dust from a demolished warehouse.”

Or, as he puts it more simply elsewhere, “Shake a tree, and things fall out.”