Noroviruses. Photo by Charles D. Humphrey, Centers for Disease Control and Prevention
Noroviruses. Photo by Charles D. Humphrey, Centers for Disease Control and Prevention

An Infinity of Viruses

When I talk about viruses, I have to struggle with big numbers.

If you get sick with the flu, for example, every infected cell in your airway produces about 10,000 new viruses. The total number of flu viruses in your body can rise to 100 trillion within a few days. That’s over 10,000 times more viruses than people on Earth.

If there can be so many viruses in a single person, how many viruses are there in total on our planet? I’ve hunted around for a number, and the one I’ve seen most often is 10311031. As in, 10000000000000000000000000000000. As in over 10 million times more viruses than there are stars in the universe. As in, if you were to stack one virus on top of another, you’d create a tower that would stretch beyond the moon, beyond the sun, beyond Alpha Centauri, out past the edge of the Milky Way, past neighboring galaxies, to reach a height of 200 million light years.

Now it seems that I may have been lowballing that number.

Scientists don’t come up with a number like 1031 by counting every single virus on Earth. They take surveys in different environments–the soil, the water, the ocean floor–and then extrapolate. As they take more samples, they can adjust their estimate up or down.

It used to be that scientists could only conduct these surveys by squinting down a microscope and actually spotting the individual viruses. That turns out to be a bad way to count viruses, in part because their hosts–such as bacteria and protozoans–often don’t grow in the artificial confines of a laboratory. Without viable hosts, the viruses can’t reproduce, and so they go unnoticed.

Things got a lot better once scientists found a new way to count. Instead of looking for full-fledged viruses, they just looked for pieces of their DNA. The ocean, once considered a desert for viruses, turned out to be a broth of viral DNA. Given the sheer volume of the oceans, most of the world’s viruses reside there. And most of the viruses that scientists discovered in the ocean turned out to be parasites of bacteria, known as bacteriophages.

But scientists have long known that there’s another kind of virus out there, one that uses a somewhat different molecule for its genes.

DNA is a double helix, which encodes genes along both strands. When our cells make a protein from a gene, they make a copy on a single-stranded molecule called RNA. DNA-encoded genes can also make RNA molecules that do other jobs, such as sensing the concentration of various elements inside a cell.

In some viruses, RNA also takes on DNA’s job, and encodes genes. Influenza is just one example of the many RNA viruses out there (or in you). Unlike bacteriophages, RNA viruses almost never infect bacteria. Instead, they infect us, plants, fungi, protozoans–all members of the same great lineage of life known as eukaryotes.

The methods that scientists use to tally up DNA viruses often miss RNA viruses, so it’s been hard for scientists to estimate how many RNA viruses are on Earth. In the current issue of the ISME JournalIn the current issue of the ISME Journal, scientists at the University of Hawaii describe a new method they developed to fill that hole.

The scientists first scooped up water from off a pier on Oahu (about 115 liters all told). They then passed the water through fine filters to exclude bacteria and larger organisms. Through a series of additional steps, they concentrated the viruses down further and then extracted all the DNA and RNA from their samples. The scientists then measured how much of each kind of molecule was present in the sea water. Based on the mass of RNA in an average virus, they could then estimate how many RNA viruses were in the sample.

Their conclusion was remarkable: RNA viruses made up between 38 and 63% of the viruses in the sea water. In other words, about half of the viruses in the ocean are RNA viruses.

This can be surprising when you consider the number of hosts that RNA and DNA viruses can infect. In the ocean, bacteria (hosts to DNA-based bacteriophages) far outnumber eukaryotes (hosts to RNA viruses).

But it’s important to bear in mind that eukaryotes are better as virus incubators. A single eukaryote cell can spew out far more viruses than a single bacterium. Bacteria can’t churn out viruses the way you can, my dear eukaryotic reader.

This new study is just a first cut at estimating the number of RNA viruses on Earth. It’s possible that for some reason the waters off the coast of Hawaii are weirdly good places to find them. The scientists themselves acknowledge that they may be overestimating the number of RNA viruses through some flaw in their methods. But when I contacted an expert on global virus surveys, Curtis Suttle of the University of British Columbia, for his opinion, he gave a thumbs-up. “I think it is an interesting and provocative paper,” he told me. “We will have to see if it holds up.”

If it does, I’ll have to find some new metaphors to describe 10000000000000000000000000000000 + 10000000000000000000000000000000.

[For more on viruses, here’s a recent talk I gave, and here’s my book, A Planet of Viruses.]


*Some people say viruses are truly alive, and some say they aren’t. But if they aren’t, then we need some name that can encompass the things that are “really” alive, along with the things that carry genes, can evolve, and carry out all the other life-ish tasks that viruses do.