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MLA Full: "Are Viruses Necessary?" YouTube, uploaded by vlogbrothers, 27 August 2019,
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In which John gets sick and considers the role that viruses have played in his life while watching the INCREDIBLE show Journey to the Microcosmos:

I am not sure which virus is currently hijacking my cells, but I'm going to go to bed now and hope it improves. Thanks for being here.

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Good morning, Hank. It's Tuesday. 

I have a cold and it's super unpleasant. In my younger and more vulnerable years, having a cold was like a slightly annoying background hum and now it's a borderline catastrophe

I just feel horrible; all of which led me to wonder whether viruses are, strictly speaking, necessary

So there are a lot of viruses in the world-- like in a healthy river there are often over 400 million viruses in a single teaspoon of water. If you get the flu, you might have over a hundred trillion flu viruses inside of your body. There are more viruses on earth than there are grains of sand.

I don't know which particular virus is causing my cold because over 200 viral strains can cause colds, but whatever it is, there are more of them inside of my body right now than there are stars in the Milky Way.

That's a fun thing to think about when you're a germophobe.

But it gets worse or possibly better depending on your perspective because like all mammals I don't just contain viruses, I am part virus. A bunch of human DNA-- something like five to eight percent of our total genome-- comes from viruses which bears a bit of deeper explanation.

So viruses cannot replicate on their own which is one of the reasons they're often seen as not being "alive." Instead, viruses rely upon the cells they infect to make copies of themselves. Viruses insert their genome into the host cells' DNA and then the host cells start making the proteins that the virus needs to make more of itself.

That's what's happening right now. A virus is using a bunch of my host cells to make more of itself and there's a battle going on and eventually, I will win that battle. Hopefully.

But sometimes a virus will infect a reproductive cell-- a sperm or an egg-- and if that sperm or egg goes on to, you know, to create an organism the viral changed DNA will be in all of that organism's cells. And the viral proteins that are now being regularly produced by this new organism will sometimes be used by that organism for new tasks because from an evolutionary perspective it's a lot easier to use proteins that your DNA is already making than it is to, like, invent entire new ones. 

This process was, for instance, critical to the development of placentas. Like, over a hundred million years ago, there was this small shrew like creature that developed a placenta using proteins from a virus. Without that virus, placentas would not have evolved as they did, humans would not exist as they do, and I would not be able to be miserably sick on account of how I would not exist.

So there are two takeaways for me here: one, I cannot unambiguously hate viruses because I am partly a product of them and two, life is weird. Like, really, really weird. 

Hank, you've helped me get my head around the extraordinary diversity and ubiquity of life on earth with your new show Journey to the Microcosmos, which is maybe my favorite YouTube channel of all time.

Like, not since I watched Carl Sagan host Cosmos as a kid have I felt such awe toward the strange universe around us. Zoom way out and we're just one solar system among billions of solar systems in a galaxy that is among billions of galaxies. But zoom way in and look at, for instance, how microscopic organisms eat and hunt-- and a teaspoon of water becomes a universe of its own.

And whether it comes to defining life or fathoming our place in the world, looking at the microcosmos can be especially interesting. Because as you said in a recent video, when you look very, very close, every sharp line turns out to be blurry. 

Now there aren't viruses in Journey to the Microcosmos because even by microscopic standards viruses are very small. Also, based on the photographs I've seen of viruses, I'm not sure I need to see video of them. But Hank, your show has helped me to see the microscopic world as something more than a mere threat and I really appreciate that.

All right, go watch Journey to the Microcosmos; it's so good! I'm gonna go to bed. 

Hank, I will see you on Friday.