More than a quadrillion quadrillion individual viruses exist on Earth, but most are not poised to hop into humans. Can we find the ones that are? 

An estimated 10 nonillion (10 to the 31st power) individual viruses exist on our planet—enough to assign one to every star in the universe 100 million times over.

Viruses infiltrate every aspect of our natural world, seething in seawater, drifting through the atmosphere, and lurking in miniscule motes of soil. Generally considered non-living entities, these pathogens can only replicate with the help of a host, and they are capable of hijacking organisms from every branch of the tree of life—including a multitude of human cells.

Yet, most of the time, our species manages to live in this virus-filled world relatively free of illness. The reason has less to do with the human body’s resilience to disease than the biological quirks of viruses themselves, says Sara Sawyer, a virologist and disease ecologist at the University of Colorado Boulder. These pathogens are extraordinarily picky about the cells they infect, and only an infinitesimally small fraction of the viruses that surround us actually pose any threat to humans.

Still, as the ongoing COVID-19 pandemic clearly demonstrates, outbreaks of new human viruses do happen—and they aren’t as unexpected as they might seem.

To better forecast and prevent outbreaks, scientists are homing in on the traits that may explain why some viruses, and not others, can make the hop into humans. Some mutate more frequently, perhaps easing their spread into new hosts, while others are helped along by human encounters with animals that provide opportunities to jump species.

When it comes to epidemics, “there are actually patterns there,” says Raina Plowright, a disease ecologist at Montana State University. “And they are predictable patterns.”

Crossing the species divide

Most new infectious illnesses enter the human population the same way COVID-19 did: as a zoonosis, or a disease that infects people by way of an animal. Mammals and birds alone are thought to host about 1.7 million undiscovered types of viruses—a number that has spurred scientists around the world to survey Earth’s wildlife for the cause of our species’ next pandemic. (Bacteria, fungi, and parasites can also pass from animals to people, but these pathogens can typically reproduce without infecting hosts, and many viruses are better equipped to cross species.)

To make a successful transition from one species to another, a virus must clear a series of biological hurdles. The pathogen has to exit one animal and come into contact with another, then establish an infection in the second host, says Jemma Geoghegan, a virologist at Macquarie University. This is known as a spillover event. After the virus has set up shop in a new host, it then needs to spread to other members of that species.

Exact numbers are hard to estimate, but the vast majority of animal-to-human spillovers likely result in dead-end infections that never progress past the first individual. For a new virus to actually spark an outbreak, “so many factors need to align,” says Dorothy Tovar, a virologist and disease ecologist at Stanford University.

For a virus, one of most challenging aspects of transmission is gaining entry to a new host’s cells, which contain the molecular machinery that these pathogens need to replicate. This process typically involves a virus latching on to a molecule that studs the outside of a human cell—a bit like a key clicking into a lock. The better the fit, the more likely the pathogen is to access the cell’s interior. SARS-CoV-2, the coronavirus that causes COVID-19, engages with the protein ACE2 to enter cells in the human airway.

For any given host, “there’s a very small number of pathogens that are able to” break into its cells this way, Sawyer says. The vast majority of the viruses we encounter simply bounce off our cells, eventually exiting our bodies as harmless visitors.

Source: Nationalgeographic