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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!
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