Epidemiologists are racing to understand and contain Middle East Respiratory Syndrome (MERS), the sometimes-lethal viral contagion spreading beyond its epicenter in Saudi Arabia. Many of them believe the MERS virus has proliferated in dromedary camels, and likely has its origins in bats.

This week, U.S. health officials disclosed that a second American had been diagnosed with MERS—the first, in April, was a Florida health care worker who had recently been in Saudi Arabia; the second, just announced Monday, is another Saudi Arabia-based health care worker who was visiting family in Florida. He was hospitalized May 8 in Orlando, and officials there said he remains in isolation and is improving.

For his recent book Spillover, National Geographic contributing writer David Quammen spent five years researching what he describes as an increasingly threatening form of infectious disease: zoonosis. That means human illness caused by viruses, bacteria, or other pathogens that have made the leap from animals to people.

Quammen recently answered some urgent questions raised by the MERS epidemic.

Why do some of these viruses become so destructive in human beings? Severe Acute Respiratory Syndrome, which killed nearly 800 people in 2003, was determined to be a virus “spillover” from bats to people. But the bats themselves didn’t seem to be suffering from SARS or anything like it, right?

In a “reservoir host,” like those bats, a virus can live harmlessly over the long term. It can replicate, but it doesn’t cause cataclysmic disease. That’s probably a result of a long association—in the case of bats, maybe a million years. But then it spills over into a new creature. It’s got a new situation. And either it discovers this is a habitat in which it just can’t live, or it’s a habitat in which it can live but doesn’t cause terrible disease.

Or, the third possibility—it discovers, “Wow! Here is a host in which I can thrive. I can replicate, I can cause all kinds of damage, and maybe I can even pass between one of these kinds of creatures and another”—one human and another. In that case, the virus has won the sweepstakes.

What sets off these spillovers—the initial spread from animal to person?

Opportunity. Some human comes into contact with an animal that’s carrying a virus. It seems to be happening more in recent decades, probably for two reasons: First, we’re better able to identify and track these diseases. Second, we’re increasingly coming into contact with wildlife. More and more, we humans are going into tropical forests and the other really diverse ecosystems on the planet. We’re cutting trees, killing animals, building timber camps, building mines, causing destruction—and we’re offering the viruses that live in the wild animals of these places the opportunity to get into humans, and then pass from one human to another.

What exactly is a pandemic? Saudi and international health officials say MERS is not one—that this is what they are working so hard to avoid. What would have to happen—let us ardently hope it does not—for the “pandemic” label to be applied?

You have three categories. An outbreak is a localized event. An epidemic usually suggests something that has spread across a country. Apandemic is something that has spread quickly and broadly around the world. So far, there’s only a sprinkling of MERS cases outside the Arabian Peninsula, and there are very careful public health steps being taken to control it.

This is a virus that bears very close watching, very rigorous measures to contain it. The greater the number of people it gets into, the greater the chance it can mutate into something new. That’s why it’s very important to control MERS at the beginning, so it doesn’t have a chance to build up high levels of virus in a large number of people, giving it more chances to mutate. The current thinking among scientists and health officials seems to be that we should be highly wary—but not panicked.