Name: なぜこのウイルスは伝染性が高いのか？ (Why Is This Virus So Contagious?)
Uploaded: 2021-01-14T10:58:13.000Z
Duration: 7 min 57 s
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Thanks to Brilliant for supporting this episode of SciShow.

Go to Brilliant.org/SciShow to learn more about their course on Logic.

This episode was filmed on April 28th, 2020.

If we have more recent episodes on COVID-19, we will include them

Scientists have learned a lot about the virus that causes COVID-19

since its discovery a few months ago, but one of the big lessons

is that people with it are super contagious.

For instance, on April 7th, the US Centers for Disease Control

published an analysis which estimated that, on average,

each person who caught the virus in Wuhan may have infected

That's about twice the estimate for SARS back in the early 2000s.

And, keep in mind that outbreaks grow exponentially, not linearly.

So if one person with SARS infects three others, and each of them

infect three, and so on, after five rounds, the disease has spread

But if a person with COVID-19 infects about six people,

who each infect five or six people, et cetera—after those five rounds,

We don't know all the reasons this disease is so contagious.

Researchers around the globe are still piecing together clues.

But by comparing this new virus to the related virus that causes SARS,

they've found some promising leads—and those leads might help us

figure out how to actually beat this thing.

How contagious a disease is depends on a lot of things,

Like, we know that one big thing that makes the

new coronavirus stand out is that people can pass along the infection

This is what you might have seen referred to

And with this virus, there also appears to be at least some

asymptomatic transmission: people passing it along

For many diseases—including SARS—those kinds of transmission

The short answer is that those older diseases need

relatively large droplets of fluid from the airways

to successfully leap into someone else—the kind only expelled

Meanwhile, this new virus may be making the jump on smaller droplets,

like the ones made by talking or breathing.

And if that's true, we're not entirely sure how it pulls that off.

One possibility is that people who are infectious have a ton

of virus particles in them, or a high viral load.

If their throat and nose contain a lot of viruses,

then even the smaller droplets they breathe out

could contain enough viruses to infect someone—

or what epidemiologists call the infectious dose.

Now, you might think you'd know if your body is chock full

of viruses—you'd assume you'd have, like, symptoms.

But that's not necessarily the case here.

With many respiratory viruses, including the original SARS,

the symptoms actually come from your immune system's reaction

And researchers are finding that people with COVID-19 can have

really high viral loads even though they aren't super sick.

In fact, the timing of peak viral load, especially in the nose

and throat, seems to be really early on, like around

With that virus, peak viral load occurred about ten days

That suggests the COVID-19 virus can infect cells and replicate

more quickly—or, in virology terms, it is a fitter virus.

So, essentially, this virus is more efficient at virusing

Which might seem strange since the two viruses are so alike.

I mean, they even get into cells the exact same way.

Both hitch a ride in on protein on our cell membranes

called angiotensin converting enzyme 2 or ACE2.

Normally, these enzymes play a big role in maintaining blood pressure,

so they're found on a lot of cells, especially ones

And both viruses use a specific protein to grab onto ACE2.

It's called the spike protein because, well, it looks like

a spike sticking out of the surface of the virus.

But what differs between the SARS virus, SARS-CoV-1, and this virus

Researchers estimate that this newer coronavirus binds

And that's probably because it has a number of changes

to the part of the spike that actually binds with the enzyme.

In fact, about half of the amino acids in this particular region

This matters because the better a virus is at binding to its receptor,

the fewer viruses you need to infect a cell.

And ultimately, that may mean the infectious dose

is actually lower for this virus than SARS

If so, that could also help explain why people can spread this virus

before they're really ill, and why it can travel in smaller droplets.

It's back to that numbers game—if it takes fewer viruses

to infect someone, then even small exposures to the virus

But there may also be other things helping to lower

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なぜこのウイルスは伝染性が高いのか？ (Why Is This Virus So Contagious?)

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