Viruses and the Virome

Our Balancing Act with Viruses

From the depths of the ocean to the tops of mountains, everywhere there’s life on Earth, there are viruses—including within our own bodies. 

You may have heard of the human microbiome, the communities of bacteria and other microbes that live on us and inside us. “Scientists estimate that we have roughly as many bacterial cells hanging out in or on each of us as we have human cells,” says Rob DeSalle, curator in the Museum's Division of Invertebrate Zoology.

And just as we have a microbiome, we also have a virome. Tens of trillions of amazingly diverse viruses call us home.

The ones that spring to mind first, of course, are those that cause disease. But disease is only a small part of the story. Wherever scientists look in or on our bodies, they find communities of viruses: in our blood and our guts, on our skin, in our mouths and our lungs, in our nervous and reproductive systems. These viruses vary enormously from person to person. Many are benign or helpful. Some are not even there to infect our own cells.

Viruses helped shape our evolution and make us who we are.

Given the abundance of bacteria in the human microbiome, most of our viral colonizers are probably phages: viruses that depend on bacterial hosts, infecting them and replicating within them. Sometimes phages kill their host bacteria, sometimes they add their DNA to that of the host, and sometimes they just hang out and wait for the right conditions to act. As part of a complex microbial ecosystem within us, the phages in our virome can affect the balance of bacteria in our microbiome—which in turn affects our health.

Other important members of our virome can affect us more directly, by infecting human cells. We know a lot about some of these, particularly the ones that cause disease. Some of them can make us acutely sick, while others may infect our cells without causing illness for long periods of time, or perhaps ever. 

But many of the viruses that infect human cells appear to be benign passengers. Some have been with us for thousands or even millions of years. Their ancestors infected our ancestors. They evolved with us as we evolved. And because many viruses can insert their own genetic material into their hosts’ DNA, viruses are an intimate and consequential part of our history. 

For example, mammals need a protein called syncytin to build a properly functioning placenta, which nourishes and protects growing embryos. The gene for syncytin comes from a virus that infected an ancestral mammal more than 100 million years ago. If it weren’t for that virus, we might never have been born.

Illustration of an opposum-like mammal about to eat an insect off a leaf.
An artist’s rendering of the hypothetical ancestor of all placental mammals. A virus carrying the syncytin gene may have been crucial for development of the placenta.
© C. Buell

Whenever we encounter a new dangerous virus, our immune system springs into action to recognize it and defend us, using tools our ancestors developed over the course of millions of years from their encounters with previous pathogens, including viruses. Vaccines work by alerting the immune system to be on the lookout for a particular virus or other pathogen; advance warning can make all the difference.

For good and ill, viruses helped shape our evolution and make us who we are.

Created with the support of the City of New York Department of Health and Mental Hygiene. © 2023 City of New York