With the global human population topping 7 billion, humans are, more than ever, encroaching on animal territory.

"We're cutting our way through the Congo, cutting our way through the Amazon, the forests of Southeast Asia," said science writer and National Geographic contributor David Quammen.

 

Quammen said that is why many of the scary, emerging diseases are zoonotic — that is, they jump from animals to humans. The list includes HIV, SARS, avian flu, ebola and the Marburg virus.

"As we disrupt ecosystems, kill and eat the animals that live there, we expose ourselves to their viruses, and we offer an opportunity for those viruses to leap from the animal host into us," Quammen said.

Quammen said public health researchers are betting if there is another large human pandemic, it too will start out by living in an animal host.

"What they say is if there is a next big one, a new human pandemic, it will almost certainly be a zoonotic disease, it'll be a spillover from an animal species," Quammen said. "It will almost certainly be a virus, and most likely will be a certain kind of virus, an RNA virus."

An RNA virus rather than a DNA virus, Quammen said, because RNA viruses mutate more when they reproduce, and thus evolve more quickly to human hosts.

Quammen spent five years tracing the evolution of today's zoonotic diseases for his new book, "Spillover: Animal Infections and the Next Human Pandemic."  He includes examples of HIV coming from a single Cameroonian chimpanzee.

Quammen is speaking Tuesday evening at the Academy of Natural Sciences in Philadelphia.