We kept a bottle of mosquito repellent right next to the back door this summer and made sure the balky screen door was pulled tight each time we came in from the garden. Firing up the grill in the evening called for long sleeves, long pants and proper shoes, not flip-flops.

The mosquitoes in our corner of Washington have always been thick and aggressive. But this year, with the surge in West Nile virus infections, those critters were downright menacing. The precautions we took were familiar: They were the same ones I followed when traveling to far-off tropical locales where malaria-carrying mosquitoes were a danger.

But here, literally in my back yard?

This should be no surprise, according to the tale David Quammen tells in “Spillover,” his highly engaging exploration of animal infections and the perils they pose for people. This year’s West Nile outbreak, the deadliest in the United States since the virus was first spotted in New York in 1999, represents the local front in a perpetual global struggle with what are called zoonotic diseases. These are diseases that cross from animals to humans, and they are the subject of Quammen’s inquiry and his many adventures. An intrepid author, he tracks the killers — some with storied names such as ebola, SARS and HIV, others lesser known such as Hendra and Q fever — across jungles and farms to identify the animals on which the germs hide and to chronicle how they make the leap to people.

These diseases merit our urgent concern. For one, they constitute the majority of all illnesses that strike humans. More than 1,400 human pathogens have been catalogued, and about 60 percent of them cross from nonhuman species to humans. An even higher proportion of human diseases that have emerged in recent decades, such as SARS and Lyme disease, have their origins in animals. And since the 1970s, the emergence of new zoonotic diseases has been accelerating. If we’re watching for the source of the next big epidemic, look to the animals.

"Spillover: Animal Infections and the Next Human Pandemic" by David Quammen (W. W. Norton)

As Quammen writes, such zoonotic diseases are particularly confounding because they are so hard to eradicate. Over the generations, mankind has notched only a few victories over microbes. The elimination of smallpox was one. For years, armed with a range of vaccines, global public health officials have been closing in on polio as well, with the virus continuing to circulate only in a few Asian and African countries. Neither virus infects animals. So when global vaccination campaigns finally eliminate polio in humans, it’s finished.

But diseases such as influenza and HIV live on in animals — in water birds in the former case, chimpanzees in the latter — and as long as they do, the bug can again cross to humans and ignite a new epidemic.

Identifying the species that act as reservoirs, silently sheltering the pathogens with few outward signs or symptoms, can be a challenge. Quammen accompanies researchers on their hunts time and again. In pursuit of the Nipah virus, which frequently causes fatal inflammation of the brain, he journeys to the Ganges Delta of southern Bangladesh and recounts the efforts on a moonless night to catch flying fox bats in vast mist nets for testing. In northern Bangladesh, he joins a team of researchers at a Hindu shrine atop a hillock overrun by macaques possibly infected with the herpes B virus, which can devastate the human neurological system. The team courageously corners several of the frenzied monkeys in a makeshift trap and injects them with a tranquilizer before stuffing them into a duffel bag.

In pursuit of ebola, a highly lethal disease that causes intense fevers and massive internal bleeding, Quammen roars into the forests of Central Africa in a Bell helicopter. There, he links up with a team of medical researchers trekking across Gabon, in part to study the effects on gorillas. While probing the emergence of HIV/AIDS in humans generations ago, Quammen ventures to the frontier between Congo and Cameroon and in the warrens of local markets searches for poached ape meat to reconstruct how the virus could have crossed from chimp to human hunter.

His accounts make for colorful reading. Though they can meander at times, at their best they are arresting and unnerving. And Quammen intersperses judicious helpings of science and epidemiology, enough to leaven the narrative without bogging it down.

As the book nears its close, Quammen writes that the outbreak of these diseases, whether new or recurring, is the result of human action: “We should recognize that they reflect things that we’re doing, not just things that are happening to us.”

Often, they are products of human development. As farms and pastures encroach ever deeper into the wilderness, ecosystems are disrupted and people come into closer contact with disease carrying animals. As jungles and rain forests are razed, as factory farms proliferate to produce chicken and other meat for a global middle class, as jetliners knit the skies with multiplying flights and routes, microbes have more opportunities to jump from animal to person and speed around the globe.

Public health experts suspect that West Nile virus, first identified in Uganda 80 years ago, made landfall in the United States when a mosquito carrying it stowed away on a transcontinental flight. The mosquito infected a blue jay or a crow, and birds became the reservoir for the virus.

This year, a mild winter and an unusually hot summer — which look suspiciously like results of man-made climate change — yielded a bumper crop of virus-carrying mosquitoes. The result is an unprecedented outbreak that has sickened people in almost every state. According to top West Nile experts from the Centers for Disease Control and Prevention, the disease is still in its “adolescence” and could be with us for decades.

In “Spillover,” Quammen advances an argument about the costs of human progress. The book would have benefited if he had made this case more thoroughly, using the theme of unintended consequences to stitch together chapters that at times read like a collection of dispatches from the microbial battlefront. But he does muster the argument in compelling fashion in the book’s final pages, warning that we disrupt the environment at our own peril. “We shake the trees, figuratively and literally,” he writes, “and things fall out.”

Alan Sipress is The Washington Post’s deputy business editor and author of “The Fatal Strain: On the Trail of Avian Flu and the Coming Pandemic.”


Animal Infections and
the Next Human Pandemic

By David Quammen

Norton. 587 pp. $28.95