Global Health Institute

The study, “Global Estimates of Mammalian Viral Diversity Accounting for Host Sharing,” also estimates that the total number of mammalian viruses is closer to 40,000, compared to past estimates of about 1.5 million.

“It’s actually really good news to know there’s a much smaller mountain to climb,” says Colin J. Carlson, a postdoctoral researcher in the Department of Biology and lead author on the study. “Five hundred to 700,000 viruses – there’s just no way we can name all of those. Ten thousand viruses – that’s the maximum number of Facebook friends you can have.”

Counting Viruses

Past studies suggested a linear relationship between mammals and viruses, assuming that each mammal has a set number of viruses it can contract, and that no other mammal will share the same virus.

“The 1.5 million comes from assuming that if you start with data from a bat and monkey that we know really well, you can scale the 290 or so viruses per host up without ever assuming they share a single virus,” says Carlson. “In reality, we know that’s completely untrue.”

Rabies, for example, easily transmits across mammals, and the flu can transmit from birds to mammals.

‘Viral Evolution’

“We’re thinking about the full host range of viruses, which is really what’s new in this estimate,” Carlson says. “There’s this broad connectedness in viral evolution that has been missing from the picture.”

In addition to contributing this new perspective, the paper builds on previous work, using a dataset and mathematical pattern to scale up the predicted number of viruses published in recent studies by colleagues in the field.

“This paper comes out of collaboration and people building on each other,” Carlson says.

Network Science

Carlson coauthored the study with Georgetown Ph.D. candidate Casey M. Zipfel, postdoctoral fellow Romain Garnier, and Shweta Bansal, an associate biology professor at the university.

Bansal’s lab focuses on disease ecology and network science, leveraging both bottom-up methods such as behavioral data collection and top-down techniques such as statistical modeling to understand how social behavior shapes infectious disease transmission. Bansal says,

When you think of problems as networks, you can see holistic solutions or patterns that you otherwise might not see.

“The integration of network science with empirical data, especially large-scale datasets, can improve the realism and utility of network-based findings,” says Zipfel.

The Next Decade

Researchers have only named two to three percent of the 40,000 estimated mammalian viruses, but more confidence in how many viruses are out there will help set the agenda for future research and interventions, according to Carlson.

“Knowing how many viruses there are helps us plan for the next decade of work,” he says, “which includes vaccine development or outbreak tracing or even communication and capacity building with local communities.”

Though the paper contributes substantial progress, the authors say there needs to be a continued focus on strengthening outbreak response, preparation and prevention.

“The lower number of viruses in general, and of zoonotic viruses in particular, simply allows the allocation of more resources to each potential threat,” says Garnier.

“It only takes one outbreak to devastate a community,” says Carlson. “We still have the fundamental task of keeping people healthy in the world we already live in.”