SARS-CoV-2 is just one of nonillions of viruses on our planet, and scientists are rapidly identifying legions of new species.

Mya Breitbart has hunted novel viruses in African termite mounds, Antarctic seals and water from the Red Sea. But to hit pay dirt, she has only to step into her back garden in Florida. Hanging around her swimming pool are spiny-backed orbweavers (Gasteracantha cancriformis) — striking spiders with bulbous white bodies, black speckles and six scarlet spikes that make them look like a piece of medieval weaponry. Even more striking for Breitbart, a viral ecologist at the University of South Florida in St Petersburg, was what was inside. When she and her colleagues collected a few spiders and ground them up, they found two viruses previously unknown to science.

Although we humans have been focused on one particularly nasty virus since early 2020, there are legions of other viruses out there waiting to be discovered. Scientists estimate that there are about 1031 individual viral particles inhabiting the oceans alone at any given time — 10 billion times the estimated number of stars in the known Universe.

It’s becoming clear that ecosystems and organisms rely on viruses. Tiny but mighty, they have fuelled evolution for millions of years by shuttling genes between hosts. In the oceans, they slice open microorganisms, spilling their contents into the sea and flooding the food web with nutrients. “Without viruses,” says Curtis Suttle, a virologist at the University of British Columbia in Vancouver, Canada, “we would not be alive.”

There are just 9,110 named species listed by the International Committee on Taxonomy of Viruses (ICTV), but that’s obviously a pitiful fraction of the total. In part, that’s because officially classifying a virus used to require scientists to culture a virus in its host or host cells — a time-consuming if not impossible process. It’s also because the search has been biased towards viruses that cause diseases in humans or organisms we care about, such as farm animals and crop plants. Yet, as the COVID-19 pandemic has reminded us, it’s important to understand viruses that might jump from one host to another, threatening us, our animals or our crops.

Over the past ten years, the number of known and named viruses has exploded, owing to advances in the technology for finding them, plus a recent change to the rules for identifying new species, to allow naming without having to culture virus and host. One of the most influential techniques is metagenomics, which allows researchers to sample the genomes in an environment without having to culture individual viruses. Newer technologies, such as single-virus sequencing, are adding even more viruses to the list, including some that are surprisingly common yet remained hidden until now. It’s an exciting time to be doing this kind of research, says Breitbart. “I think, in many ways, now is the time of the virome.”

In 2020 alone, the ICTV added 1,044 species to its official list, and thousands more await description and naming. This proliferation of genomes prompted virologists to rethink the way they classify viruses and helped to clarify their evolution. There is strong evidence that viruses emerged multiple times, rather than sprouting from a single origin.

Even so, the true range of the viral world remains mostly uncharted, says Jens Kuhn, a virologist at the US National Institute of Allergy and Infectious Diseases facility at Fort Detrick, Maryland. “We really have absolutely no idea what’s out there.”

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