Twleve years ago, David Quammen sat at a campfire in a Central African forest, listening to two Bantu men talk about an outbreak of Ebola in their village. What they told him was that at the same time the dreaded virus had been killing their loved ones, it was also slaughtering gorillas in the jungle nearby. Thirteen dead gorillas, in a pile, paralleled the human toll of 21 dead.

This became Quammen’s point of entry to a truth that some scientists already knew well: Most of the infectious diseases that afflict humans — causing weird outbreaks, epidemics, and in some cases global pandemics, with millions dead — come to us from wildlife. Any such sudden transfer of disease from one species to another is known as a “spillover.” Quammen’s new book, “Spillover: Animal Infections and the Next Human Pandemic” (W.W. Norton & Co., $28.95), traces the science, the history and the human pathos of the subject for a popular audience.

“The phenomenon of spillover hasn’t been much noted in the media because people (including reporters) tend to think of human medical concerns and ecological considerations separately,” explained Quammen. “Medical reporters don’t know much about chimpanzees or bats in the wild. But the reality is that human health, the disruption of wild ecosystems, the infection status of bats and rodents and monkeys, and the ecology and evolution of viruses are all inextricably connected. What ‘Spillover’ offers, which hasn’t been offered to a popular audience before, is the synthesis of all those subjects and concerns.”

The AIDS pandemic resulted from a spillover — one chimpanzee passing the virus to one human, in southeastern Cameroon, around 1908 — and Quammen tells that story as it has never been told before. According to the author, the next big and murderous human pandemic, the one that kills us in millions, will be caused by a new disease — new to humans, anyway. The bug that’s responsible will be strange, unfamiliar, but it won’t come from outer space. Odds are that the killer pathogen — most likely a virus — will spill over into humans from a nonhuman animal.

“Human population now stands at 7 billion,” said Quammen. “We are traveling around the world, and moving other creatures (including livestock, wild animals and the infectious microbes they carry) around the world, faster and more abundantly than ever before. No other large-bodied species of vertebrate animal has ever been so abundant on Earth — we know that from fossil records. The human population is like a very, very dense forest, which has become very, very dry, and which is now awaiting the spark from a lightning strike. Any new virus could be that spark. Bubonic plague killed a third of the population of Europe, and the 1918 influenza killed 50 million people worldwide, despite the fact that in those days the forest wasn’t yet nearly so dense and so dry.”

“Spillover” is a starling book that delivers news from the front lines of public health. It makes clear that animal diseases are inseparable from us because we are inseparable from the natural world.

Contact staff writer Bobbi Booker at (215) 893-5749 or This email address is being protected from spambots. You need JavaScript enabled to view it. .