It's rare to see a positive headline about viruses these days — or even a neutral one. From the yellow fever pandemic of 1793 to the COVID-19 outbreak that started here in 2020, viruses only make the news in the United States when they are killing people or, at the very least, stirring up a lot of fear.
That is, perhaps unfortunate, because most of the viruses inside you are benign. Oh, and about those viruses inside you: there are a lot of them. And they are absolutely everywhere. If you think this article will give you advice on how to get rid of them, you are out of luck, because you have more viruses on or inside of you body than you have actual cells.
In fact, according to the scientific journal Nature, there are 10 to the 13th power — that's a 1 with 13 zeroes, or 10,000,000,000,000 — virus particles per human. Welcome to your virome. Believe it or not, it is not a bad thing for you to have that many.
"There is the devastating negative aspect that viruses cause a number of illnesses as we are seeing now in COVID-times," Dr. Travis Thomson, an assistant professor at the University of Massachusetts Medical School who specializes in junk DNA and indirectly endogenous retrovirus control, told Salon by email. "However, the earth's biome needs viruses for its survival."
If you read stories about how climate change could lead to more pandemics, that is because the oceans are teeming with the "living" creatures.
"Viruses are needed for the breakdown of all organic material in all ecosystems on, above, or in the Earth."
"If viruses were not overturning — i.e., killing hosts in the ocean — there would be a huge deposition of detritus on ocean floors, which is needed for all life on the planet to survive," Thomson wrote to Salon.
He added that this is not just the case for the ocean. "Viruses are needed for the breakdown of all organic material in all ecosystems on, above, or in the Earth," Thomson explained. "Then there are the viruses that live in us: endogenous retroviruses, or transposons, that my group and others are discovering have functions in helping us carry out many physiological functions such as fighting off infections and even for proper neuronal development."
Of course, it is not enough to simply say that the human body is brimming with viruses. A virus, after all, cannot "live" in any old organ of the body. By definition, a virus is a simple biological entity with a protein shell and genetic material (an RNA or DNA strand) as their core. They exist and reproduce by infecting living cells, infiltrating their nuclei and forcing the cells to manufacture copies of the virus. This means that the virome, though massive, is still confined by the fact that the viruses themselves must be involved with our cells.
As a result, the human virome is divided into two separate realms, some of which infect our own cells, and some of which affect the cells of other things that live inside us — things like gut bacteria and the like.
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"The human virome (and the virome of any multicellular organism) consists of two major, distinct components," explained Dr. Eugene Koonin, a biologist and Senior Investigator at the National Center for Biotechnology Information. Koonin wrote to Salon that there are viruses which reproduce inside a human being's own cells, and then there are the viruses that infect the bacteria and archaea which can be found in our microbiome.
"Both components are important in their distinct ways," Koonin observed. The ones that infect cells, as anyone who has gotten sick can tell you, are obviously capable of doing very bad things to us. At the same time, "there is also a healthy human virome that consists many diverse viruses reproducing in our cells without detectable pathological consequences." Many of them leave behind genetic material known as junk DNA that can be repurposed by our own genome to help perform important functions. In one famous example, an ancient viral gene known as Arc is believed to be responsible for helping humans encode information (i.e., "learn"), form memories and in other ways perform processes essential to conscious thought.
Similarly, viruses can act as human body guards without even realizing it. When viruses infect your cells, it is not personal; they simply want to reproduce. If they can do that using the cells of other microbiota inside of you rather than you personally, that works just fine.
"The more we sequence the more we realize there is a spectacular amount of complexity of viruses, and how they interact with us both for the good and the bad, to be discovered."
"Viruses infecting bacteria and archaea in our microbiomes regulate the population size of their hosts and through that can have various indirect effects on human health," Koonin told Salon.
If there is any problem with the human virome, it is that we know as little about it as we do about the universe beyond Earth's atmosphere. According to Dr. Jason D. Shepherd, an associate professor of neurobiology at the University of Utah School of Medicine, it is downright "unexplored."
"We only just determined that bacteria have important functions in normal physiology," Shepherd explained by email. "There's an emerging idea that transposable elements, which are virus-like, are critical for normal development. Part of the hurdle has been detecting and sequencing viral genomes from cells. It's also not clear if viruses can regulate normal physiology... the lines are blurred when we consider that there are virus-like proteins expressed in the body that may work just like viruses (e.g., our work on Arc)."
This lack of virome knowledge also means that scientists are more likely to be caught off-guard if a normally innocuous virus in our bodies suddenly turns on us.
"Some members of the healthy virome, however, can be triggered by various, only partially understood factors and become pathogens," Koonin told Salon. "Herpesviruses and papillomaviruses are probably best known examples. Other viruses in the healthy virome apparently can protect the host from infection by their more aggressive relatives."
There is good news, though: Scientists have already developed the technology to sequence viral genetic material. It may take a while, but humanity is slowly but surely beginning to understand its own internal ecosystem.
"The type of sequencing (DNA and RNA) needed to identify the complexity of all the viruses that live in the world around us, and as well as in us has just been developed," Thomson told Salon. "The more we sequence, the more we realize there is a spectacular amount of complexity of viruses, and how they interact with us both for the good and the bad to be discovered."