"We actually know from previous studies and from what's happened in the past that most viruses come from mammals ... not birds and not reptiles or other species," said Kevin Olival, lead author of the study and an associate vice president for research at EcoHealth Alliance, a global environmental health nonprofit.
To understand what might create an unexpected pandemic risk, Olival and his colleagues tried to address several pieces of this puzzle.
"So the first piece is, really: What makes some species in the world better hosts for viruses or zoonotic viruses than other species?" he said. Some animals are better reservoirs -- or sources -- of viruses than others.
Pieces of the puzzle
He and his colleagues created a database that included 754 mammal species -- 14% of the mammal diversity on the planet -- and 586 unique viral species, every recognized virus found in mammals.
Next, they analyzed 2,805 mammal-virus associations. For instance, about 45% of the total 586 viral species have been detected in humans, and of those, nearly 72% have been detected at least once in a human and at least once in another species.
Applying mathematical models to the database, the researchers discovered that the risk of a virus leaping from a mammal to a human varied based on how related the animal species was to humans as well as the frequency of contact with humans.
Yet there was also an entirely independent variable. Simply belonging to one category of animals as opposed to another made it much more likely that a particular creature might be a source of zoonotic viruses. For example, bats may not be closely related to humans, but they carry a "significantly greater proportion of zoonotic viruses than any other mammals," said Olival.
The researchers also used the database and mathematical tools to predict the number of viruses each specific animal might carry.
The second piece of the puzzle was geography: Where might unknown zoonotic diseases be hiding? Naturally, the location of each zoonotic virus' animal host answered this question. Zoonotic viruses arising from bats would cluster in South and Central America and parts of Asia, while primate viruses would be found in Africa, Central America and Southeast Asia. Zoonotic viruses carried by rodents would mostly be found in North and South America and Central Africa. Olival and his colleagues mapped this information.
The third and final piece of the puzzle was "what makes some viruses more likely to jump into people than others," Olival said. "I think that's a really important piece, and there's a lot more work that can be done, but we really provided a framework to move that predictive field forward."
Overall, Olival said, the new study, which took about six years to complete, "provides a roadmap for where in the world we should prioritize for finding and stopping the next emerging virus, and that road map includes what species we should focus on and what locations are most important and what viruses should we be looking out for."
Amesh Adalja, a spokesman for the Infectious Disease Society of America, said the research is "very important."
"We are going to continue to have infectious disease threats, and it's basically one of the great scientific and medical challenges to keep pace with that," said Adalja, also a senior associate at the Johns Hopkins Center for Health Security who was not involved in the study.
Not everyone risks catching a zoonotic infection. Some people -- including hunters, people who work in slaughterhouses and those who work in agricultural settings -- are "basically at the leading edge of the human-animal interface," Adalja said, and so are more likely to become infected with a zoonotic virus.
For this reason, there's a lot of interest in doing active surveillance of these populations, he said.
Generally, the new study tries to develop "some type of analytical way of assessing the risk of these types of infections," he added, and "does, hopefully, make this a much easier prospect."
"As human beings expand their reach and live in many different areas where they traditionally haven't lived in, you will see more opportunities for novel viruses to appear," Adalja said.
"You can think about Lyme disease as a zoonotic infection," he said, observing that Lyme became a big health issue only when people started to live in places "that are more wooded."
Olival said human behaviors are a key factor in the emergence of these zoonotic infectious diseases.
"Anything that puts us into contact with these mammal species that most people have never heard of, right, that might be living deep in the forest," he said. "So things like deforestation and road building and hunting -- all those factors are good examples of interactions that can actually cause diseases to come out of their natural habitat and jump into the human population."
Though contact makes a difference, so too does temperature. Certain viruses, such as flu, are more likely to spread in cold conditions as opposed to hot, Adalja said.
"We know, for example, that diseases like yellow fever, which people now, in today's world, think of as a tropical disease, used to occur in Boston," he said, explaining that it is also known that "mosquitoes have thrived in areas that aren't traditionally associated with mosquitoes."
During Washington's presidency, Adalja said, a yellow fever outbreak in Philadelphia
basically shut the whole government down because no one could meet in the former capital city.
And while it's true there's a lot more diversity of animal species in jungle or rain forest areas, there's a danger in believing that emerging diseases could happen in only some parts of the world. "Flu viruses might do better in colder temperatures, but other viruses might do better in tropical climates," Adalja said.
"The world is a very, very small place," he said. "It's very easy to be in a jungle in Africa or South America or the Indian subcontinent one day and in a Western city the next day."
This speed of travel allows an infection from one side of the globe to appear on another side when, in the past, it would not have been possible for an infected person traveling that far to make the trip, Olival noted.
"My biggest fear is that a disease may emerge anywhere in the world, even in the most remote region of the world, and can arrive to a major US city in less than 24 hours," Olival said.
The best way to handle this potential infectious threat is to really home in on the viruses that are at the highest threat level and then try to intervene once you've identified that, Adalja said.
"We have to continue to refine the way we basically sift through all these viruses on this planet and understand which are most likely to cause" emerging infectious diseases, he said. "So you can be much better prepared for infectious diseases and maybe even prevent them from occurring."