For Holmes and his colleagues, one insight into how and why cross-species transmission happens came from their observation that RNA viruses (which use RNA as a genetic material) seem to jump species much more frequently than DNA viruses (which use DNA) do. “That’s probably because they have a higher mutation rate,” VandeWoude said. For RNA viruses, the combination of a generally smaller genome and a higher mutation rate makes it more likely that they can adapt to a new host environment.

Holmes pins the trend to the different life histories of RNA and DNA viruses, too. RNA virus infections are often acute but transient, coming and going in a relatively short time — as with the flu or the common cold. That transience means the virus can miss its opportunity to be part of the diverging host species. “If you’re an acute virus, you only have an effect for a few days or weeks,” Holmes said. “Co-divergence is very hard to do on average. You’re not around long enough.”

In contrast, DNA virus infections are often chronic. When part of a host population splinters off to become a new species, it’s more likely to bring the virus along because more of the population would be infected. That increases the chances the virus can co-diverge with its new hosts.

The host’s way of life also plays a role in virus transmission and the odds of co-divergence versus cross-species jumping. “We know that host population size and density are really important in dictating how many viruses they have,” Holmes said. He used bats as an example: Bats have a penchant for carrying many different viruses, but that’s at least partly because “there are a hell of a lot of bats.” With such large populations, they’re just more likely to contract viruses. “A very simple rule of ecology is, the more hosts there are, the more virulent things they can carry,” Holmes said. “The chance of a virus finding a susceptible host is just higher.”

A 1975 Science paper from Francis L. Black of Yale University provided some insight into how human diseases are affected by a host’s population dynamics. Looking at fairly isolated, small societies of native Amazonian people, researchers found that although they could often detect chronic viral infections, acute infections were largely absent. Isolation kept the tribes protected from new viruses. The few viruses that did get in, if they were acute, quickly worked their way through the small tribes and died out. Without lots of hosts to sustain them, the viruses disappeared quickly.

The Risk of New Diseases

The discovery that cross-species transmission has occurred so often might seem worrisome, given its association with harsh emerging diseases. With so many jumps having occurred in the past, does the future hold more of the same?

Not necessarily. “Historical jumping rates do not necessarily predict the future, especially when it comes to humans,” Pennings said. The way we live today is so different from how humans lived just a few centuries ago, our risk of emerging diseases is probably different as well.

Humans carry a lot of viruses as well. We too have large populations, and we’re incredibly mobile, meaning we bring along viruses to new, susceptible hosts quite easily. “We have all kinds of behaviors that put us at risk for all kinds of things because we like to go poke around in places where we probably shouldn’t, we take lots of risks, we eat things we probably shouldn’t eat,” VandeWoude said. “We’re probably the worst offenders and probably the biggest target for cross-species transmission just because we do so many crazy things.”

And doing those crazy things often leads us to bump against other species. The more we do that, the more we expose ourselves to new viruses, and the species putting us at greatest risk are the ones we interact with the most. “We’re more likely to get something from rats than from tigers,” Pennings said.

But further studies of viral evolutionary history may help scientists figure out whether there are species to which we should pay more attention as sources of new infections. (Epidemiologists already closely monitor viruses at risk for passing from poultry to humans because of fears about bird flu.) Maybe viruses from plants, fish and mammals are equally risky to humans, or maybe those researchers trying to predict the next epidemic can narrow their focus to a select high-risk group.

Holmes has a different view. “I don’t think prediction is in any way viable at all,” he said. “I understand why it’s done, but what I think I get from this and the sheer number of new viruses we’re discovering is it’s just not viable.”

Luckily, it’s now much easier to do that sort of analysis with the growth of metagenomics, the study of genomic information extracted from the environment. For this study, Holmes and his colleagues pulled viral genomic sequences from a number of publicly accessible databases. They didn’t need to have physical samples of any of the viruses, which is a relatively new direction for this field. “Virology is moving into a new phase where, with metagenomics, you can now do this mass sampling stuff just to see what’s there,” Holmes said.

Holmes also noted that because it’s now so easy to access new information about viruses, the phylogenetic trees that he and his colleagues created will change a lot in the near future. “In three years’ time, these trees will be much fuller because we’ll have found so many new samples of these viruses,” he said.

This article was reprinted on Wired.com.