In a race to prevent future deadly pandemics, scientists are trying to pinpoint the animals and regions where the next Ebola or Zika might arise — before the viruses start harming people. But some experts argue that it makes more sense to look for new viruses in humans, not other animals. In fact, the next emerging infectious disease is probably already out there, making people sick.

“Right now, we’re always on defense.”

Animals host a massive number of viruses, and sometimes these viruses make the jump to humans. (These viruses are called zoonoses.) This happens pretty rarely, but when it does, it can wreak havoc: most pandemics in recent memory like HIV, pandemic influenza, and Zika were caused by viruses that started out in animals. The Ebola virus, which probably jumped from bats, killed more than 11,000 people during the recent outbreak in West Africa.

If there were a way to predict which infectious disease might emerge and threaten humans next, maybe it could give us a head start on a vaccine, or prevention strategy. But the challenge is even more complicated than you’d think, because where exactly do you go looking for the next pandemic? In animals, which carry a vast array of viruses that might never infect people? Or in humans, once a virus has made that rare leap — but before it spreads out of control?

Peter Daszak, an epidemiologist with the research and conservation nonprofit EcoHealth Alliance, wants to find these viruses before they make anyone sick. “If we allow these viruses to get into people, it’s already too late,” he says.

Where exactly do you go looking for the next pandemic?

To do that, he and his team hunted through the scientific literature to create a database of nearly 600 viruses and the more than 750 mammals they infect. Then, the researchers looked for patterns that could help them understand what makes an animal virus more likely to infect humans. Their research was published this week in the journal Nature.

A few clear trends emerged: animals that are closer to humans geographically (like rats) and genetically (like monkeys and apes) have a better shot at sharing their viruses with us. Animals harboring more virus species in general (like bats) are more likely to carry one that could sicken humans. And viruses that are transmitted by mosquitoes and can infect a broad range of species tend to hop into humans more successfully. These are trends that previous studies also identified, but finding them with this new method is a reassuring reality check, says Barbara Han, a disease ecologist at the Cary Institute of Ecosystem Studies, who was not involved in the research.

Using these patterns, the researchers then mapped where unknown zoonoses might be hiding: in Central and South American bat populations, for instance, or rodents in North and South America. These aren’t predictions about precisely where the next virus will emerge, Daszak says. But these maps could help steer research efforts like Global Virome Project, a $3.4 billion proposal supported by organizations that include the EcoHealth Alliance to sample and genetically sequence 99 percent of the viruses that could one day threaten humans. (Sequencing a virus is an early step toward attempting to make a vaccine.) “Right now, we’re always on defense,” Han says. “The important thing about figuring out where the next one is likely to happen is that it gives us a leg up.”

“A virus doesn’t just jump out of a bat and cause an epidemic in humans.”

But some researchers say that spotting the next pandemic before it starts will take more than sequencing the viruses that currently infect animals. “While most pandemics are zoonoses, most zoonoses do not cause pandemics,” infectious disease expert James Lloyd-Smith writes in a commentary published alongside the Nature paper. So, it’s important to figure out the factors that drive a virus to spread across the globe. This depends more on human-to-human than animal-to-human transmission, he writes.

That’s because there are a lot of hurdles that an animal virus has to clear before it spreads to a person, and from there, to another person. “A virus doesn’t just jump out of a bat and cause an epidemic in humans,” says Ronald Rosenberg, an infectious disease researcher. Instead, a virus can spend decades or even centuries hopping back and forth between animals and humans before the conditions come together for an outbreak. (The exception are influenza viruses, which can make this leap more rapidly, Rosenberg says.)

“We don't really have the wherewithal to identify epidemics as they're beginning in the human population.”

For example, scientists discovered the Zika virus in monkeys living in the treetops of an Ugandan forest in 1947 — decades before it caused the first large outbreak on the island of Yap in 2007, and nearly 70 years before it spread across the globe. Yet, the virus still managed to catch the world unprepared: in fact, scientists are still racing to develop a vaccine or cure. “It wasn’t really a matter of whether we could find these viruses early in animals,” Rosenberg says. “We needed to examine them more closely after they were found in humans.”

What we really need, experts like Lloyd-Smith and Rosenberg say, is better surveillance in human communities — especially in ones that frequently come into contact with wildlife. That means setting up sentinel clinics in viral hot spot regions that can screen sick patients for the usual infectious suspects. Central laboratory facilities could hunt for less typical, or completely unknown, infections, if those initial screens come up negative. Rosenberg is currently piloting such a system in Uganda.

“My biggest fear is that we don’t do anything, and we discover these viruses the hard way.”

It’s possible the next pandemic could already be infecting people, but without the time, money, or tools to identify the cause of every fever or illness, doctors and scientists might miss it. “We don't really have the wherewithal to identify epidemics as they're beginning in the human population,” Rosenberg says. “We miss opportunities many, many times every day.”

“There’s a big gap there,” Daszak agrees. “There are outbreaks that go undiagnosed.” Still, he hopes that his team’s hot spot map can help target surveillance efforts in humans, too. While more data can’t breach the financial barriers to better virus detection in people, the stakes are too high to sit still. Daszak says: “My biggest fear is that we don’t do anything, and we discover these viruses the hard way by them emerging and killing people.”