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In the past couple years, you may have found yourself wishing that all the viruses in the world just disappear. But be careful what you wish for...

Hosted by: Hank Green

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Sources:

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[♪ INTRO].

So viruses have kind of been  on everyone’s minds lately. Sure, before COVID-19, we had to deal with viruses that ranged from merely  inconvenient to devastating.

But after what has been a  truly rotten time for everyone, if someone were to come across a  supernatural wish-granting creature, you might wish for all viruses to disappear,  in the past, present, and in the future. Yeah, absolutely no one would believe your story, but at least everyone would  be free of viral misery. But based on what we know about viruses, it could be more of a monkey’s paw situation.

By sheer numbers, viruses are considered the most abundant biological entities on Earth.  That’s a lot to just wish away.   So we’re going to take a look at the  science and try to imagine what the consequences would be, for humans and for  the world, if viruses just disappeared. Ultimately, as much havoc as they’ve  wreaked throughout history, scientists are starting to understand that viruses are  a part of life on Earth as we know it. To understand what would happen if  viruses vanished, first we need to understand what a virus is.

But  that’s not a straightforward question. A lot of it has to do with how  their reproductive life cycles typically happen in two stages. The first is when a virus infects  a host and reprograms its cells.

It uses the cell’s machinery  to churn out copies of itself, often damaging or destroying  that cell in the process. Some viruses, like the one that  causes COVID-19, stick to the machinery outside of the nucleus to create copies. Other viruses enter the cell's nucleus and  incorporate their genetic material directly into the cell’s genome, and direct  the creation of copies from there.

The second is the stage outside of host  cells, when virions escape the infected cells and hang out in the external  environment, in the air, on surfaces, and more. These are virus particles, with  molecules of either DNA or RNA encased in a protein shell,  sometimes with a fatty envelope. That virus particle can’t do much of  anything when it’s just out in the world.

In fact, the idea of what a virus is is  up for debate in the scientific community. Until recently, the idea has been  that a virion in its non-living stage, just out there in the environment,  is synonymous with the virus. But some researchers think it’s time  to abandon this view, which they say is reductive because it overlooks the  way viruses transform infected cells.

This is why you hear people in the  news talking about “virus particles” in addition to the virus. Those  virus particles are the virions. Instead, that virus-making  factory within a living cell would be a more comprehensive  version of what a virus is.

Because it’s basically taking that  living cell and totally repurposing what it does. The emphasis  would be on that process. Some researchers argue that  shifting the view of the virus itself could open up new avenues  of scientific investigation.

For example, it might help us  understand where they came from. And there are three leading  hypotheses as to how viruses arose. The first, sometimes known as  the progressive hypothesis, is that they arose from pieces  of DNA or RNA that gained the ability to jump between cells.

The second, sometimes called the  reductive or regression hypothesis, is that viruses are remnants of cells that  eventually lost their ability to exist independently, forcing them to  parasitize other cells to reproduce. In the third scenario, viruses evolved  on multiple, independent occasions. In this view, they may have  first arisen before cells did, and evolved in tandem with all life on Earth.

This could have happened in  different cellular organisms, from genes that acquired the ability to  infect cells and use them to replicate. Some scientists think that any of these  different scenarios could be true for different viruses that infect  different kinds of hosts. So, for example, viruses  that infect bacteria would have come from genes that escaped from bacteria.

And viruses that infect eukaryotes,  organisms whose cells have nuclei, would have come from totally different  genes that jumped from there. In the view of viruses as virus  factories within cells, there just couldn’t be a virus before there was a cell. So no matter what you call these pre-cellular  genetic bits and bobs, scientists are starting to think that viruses have  been around as long as life on Earth.

And so there would be a lot of  hypothetical potential consequences to poofing them away, especially  if you poofed them from history. First, viruses aren’t all bad. The virus  that causes COVID-19 is plenty bad, and so it’s easy to overlook that fact.

When we think of viruses, we often  tend to think about pathogenic, or disease-causing, viruses. Those are the viruses that cause harm to  the host when they hijack its cells to reproduce, like SARS-CoV-2, and the  virus that causes smallpox, and measles. Non-pathogenic viruses, on the other hand, don’t harm hosts like pathogenic ones do.

Which is where we come to the idea of  the human virome. That is, viruses that live alongside... or inside... you,  without ever really bugging you. It’s thought to be an integral part of the  microbiome, or the collection of trillions of microbial cells in your body.

Bacteria get  a lot of the attention when it comes to the microbiome and its effect on our  health, but we have plenty of viruses doing their things too. In humans, scientists surmise that  viromes stabilize by around age 2, and that each individual’s virome has a slightly different makeup of different types of viruses. Studies of metagenomic data,  or the DNA sequences found in environmental samples, both within  and outside organisms’ bodies, have helped researchers shed  light on the makeup of the virome.

It consists of viruses that infect  bacteria, viruses that infect other microscopic organisms, and  viruses that infect human cells. As part of the microbiome, evidence  suggests that the virome plays a major role in our health. Like, recent studies of the human  virome have shown that different viruses live in different parts of  the body, like the urinary tract, gastrointestinal tract, oral  cavity, blood, and lungs.

And while we don’t fully know  yet what they’re doing there, we do have some clues. For example, a healthy mouth  has plenty of viruses helping keep the oral microbes in check. Increased presence of some  viruses that naturally colonize the oral cavity and respiratory  tract can throw that balance off and lead to various diseases,  including periodontal disease.

But even with interest in the  virome on the rise and papers piling up, we don’t really understand it yet. One thing seems to be clear:  the virome and its makeup can definitely influence our health. So, even if you could just  poof away all viruses in your body right now, you  might want to think twice.

And humans aren’t the only  animals with viromes that vary in their different tissues. All  kinds of animals, plants, and other organisms are known to have them. Not only do viruses play a role  in the health of organisms, they are also thought to shape the biosphere.

Viruses are everywhere in Earth’s  ecosystems, in big numbers. And one of the major impacts of  viruses in the environment is in how their presence affects  communities of bacteria. Based on what researchers  have learned so far, their significance is most obvious  in the ecology of oceans.

They constantly shape the diversity of natural bacterial populations under  the sea, in a very big way. They’re present in our oceans in such  astronomical numbers, it’s estimated that they’d stretch from here past the  next 60 galaxies if laid end-to-end. Every second, hundreds of trillions of  viral infections occur in the ocean.

Every day, they’re thought to kill  around 20% of all of the marine bacteria. Which, just saying, is a lot. And that’s a huge part of how nutrients  and energy cycle in the ocean.

For instance, all of those  dead bacteria are thought to release enough iron to support  the needs of phytoplankton. The composition of Earth’s atmosphere also  hinges on the shuffling of all of that organic material from viral-killing  activity in the ocean to the tune of 3 gigatons of carbon sequestered every year. And this is a very abridged list  of viruses’ oceanic activity.

Viruses’ role in soil isn’t as  well-studied as in the ocean, but the importance is clear. One gram of soil can house  up to ten billion viruses, and they can be present in  bigger numbers than bacteria. Studies suggest that viruses  may have a major impact on carbon and nutrient cycles on  land, just as they do in the ocean.

They’re also a fount of genetic  diversity in both the soil and the ocean. A lot of what we’ve talked about gets to the importance of viruses for  organisms and ecosystems right now. But here’s where we need to  remember what every time travel show and movie says about changing things in the past:.

Don’t do it, even if it seems  like a great idea in your head. Basically, if viruses did  co-evolve with life forms, then they're sort of part of life on Earth. Since viruses need cells,  researchers think the ancestry of both viruses and cellular  organisms were intertwined.

One part of that is that the trajectory of life on. Earth could have been drastically different  without viruses to infect organisms. But life on Earth is also what it is  because large parts of so many organisms’ genomes are made up of viral DNA,  including humans and other mammals.

The viruses that incorporate  their DNA into host cells’ genomes don’t just reproduce  themselves during an infection. When they infect tissues involved in reproduction, like the ones that make sperm and  egg cells, their DNA can also be passed to future generations  when the host reproduces. Scientists think this has provided lots  of raw material for evolution to act on.

In humans, for instance,  around 8% of our genomes are made up of viral sequences. Which is probably more than  you would have guessed! Throughout history, scientists think viruses have infected the cells that make human eggs and sperm, incorporating their genetic material,  which gets passed on in the genes.

Over time, those sequences can change  and even acquire beneficial functions. So, as benevolent as it might seem to wish  for viruses to cease to exist in the past present and future, definitely leave  out the past, ‘cuz if you do that, and I’m just advising you as  a person who may get a wish, if you leave it out of the past, that  would be great because if you include it, we probably won’t exist anymore. Also you might want to specify in your  wish to only eliminate the human pathogens, the ones that actually do us harm.

Because  most viruses don’t mess with us at all. Vanishing all viruses might turn out  to be a pretty short-sighted move. And there’s still so much more to investigate  about viruses, so much that we don’t know, especially now that the thinking  is shifting to viewing them not as just virions, but as everything  they do inside hosts.

We’ve even found giant viruses, that are  bigger not only in physical dimensions but the size of their genomes. And they’re a lot closer to cellular  forms of life than their smaller cousins. Their existence has led to several  hypotheses arguing that viruses aren’t non-living, an argument that’s often been made.

Instead, they suggest that viruses  are another domain of life, along with eukaryotes, bacteria, and archea. Some scientists are even hypothesizing  that they were integral to the origin of eukaryotes, including as the source of  important genes or even cellular structures. So there you have it, folks.

It might be tempting to fantasize about a  virus-free world, with all of the bad that viruses have done, killing swathes  of humans throughout history, and putting a big old damper on everything now. But the more that we learn about viruses,  the more they show themselves to be an example of how the good, the bad, and the  ugly of Nature are often inextricable. Thanks for watching this episode of SciShow, and thank you to our patron  Jeffrey R.

Fish for asking. SciShow patrons can submit questions to  our inbox, and our editors pick out the best ones to answer in episodes like this one. We don’t always get quite  this much mileage out of them, but what a fantastic question!

And you are welcome to consider this a challenge. You can get started at patreon.com/scishow. [ OUTRO ].