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  • There are an estimated...

  • this many individual viruses on Earth.

  • Luckily, there are only a little over

  • 1,000 virus strains known to infect humans,

  • the majority of which come from other animals.

  • They're part of a group called zoonotic diseases,

  • caused by bacteria, viruses, parasites, or fungi.

  • And the one we're all focusing on now

  • is the virus that causes COVID-19.

  • And new ones can emerge at any time.

  • Here's what needs to happen

  • for a virus to jump from animals to humans.

  • As a pathogen, the virus' goal

  • is to infect its host and replicate,

  • because it can't do that on its own.

  • Let's say this pig is the original host of a virus.

  • He and his buddies form a reservoir,

  • a specific population of animals of the same species

  • that naturally host a pathogen.

  • Colin Parrish: So, there are millions of viruses out there

  • infecting animals. Literally millions.

  • I mean, the more we look, the more we find.

  • And most of those viruses

  • don't infect, you know, other animals.

  • They have a restricted host range.

  • Narrator: Most of the time, the virus

  • doesn't affect its original host

  • or only mildly affects it.

  • So, what's it doing in there?

  • Suppose that this pig hosts a virus

  • that primarily infects the gut.

  • Viruses tend to attack different parts of the body,

  • depending on whether they can bind to these guys.

  • Receptors are proteins found along the outside of a cell,

  • used to communicate with the rest of the body.

  • But viruses can latch on, too.

  • Cells in a respiratory system may have different receptors

  • than cells in a digestive tract.

  • A virus does its thing by latching onto a host cell,

  • entering it or injecting bits of itself into it,

  • and then hijacking it.

  • It forces the cell to make copies of the virus,

  • all of which will go on to hijack other host cells.

  • This will usually kill the host cell.

  • And if enough of the host cells are infected

  • and make more of the virus,

  • the host will contract an infection,

  • which, if the body can't fight it off or fights too hard,

  • could lead to severe disease or death.

  • In reservoirs, however, the species has likely evolved

  • a resistance to the virus over many generations.

  • This allows a sort of equilibrium;

  • the immune system controls the infection

  • without killing the virus off completely.

  • If the virus jumps, though,

  • a new host won't have that same, or any, immunity.

  • That might sound scary, because truthfully

  • you are constantly being exposed to viruses.

  • But only a very small number

  • succeed at infecting a new host species.

  • It's called spillover,

  • and there are a series of barriers

  • that a virus must navigate for that to happen.

  • If it's held up by even one of these,

  • it can be stopped in its tracks.

  • Simplified, they represent two things:

  • Can the virus get to the new host's cells,

  • and can it bind and enter them?

  • The more infected pigs there are in one space

  • and the closer they are to people,

  • generally the more chance of spillover.

  • But the likelihood also has to do with

  • how the humans are interacting with them.

  • Animal viruses are usually transmitted to people

  • in a few ways: contact with excretions,

  • slaughter, bites, contact with tissues,

  • or through an intermediate species

  • like mosquitoes or ticks.

  • So places like farms and slaughterhouses

  • and even petting zoos,

  • where people come in close contact with animals,

  • have an increased risk of spillover.

  • Proximity alone isn't enough, though.

  • Some of it can be genetic for humans.

  • There's a huge list of genes that have been associated

  • with different risks of infections,

  • some genes offering resistance to certain infections

  • and others increasing risk.

  • Beyond genes, the virus has to get through

  • the body's innate immune responses.

  • Parrish: So, there's two types of immune responses.

  • One is the simple adaptive immune responses,

  • so those are antibodies and T cells,

  • and they generally get stimulated

  • after the infection has already occurred.

  • The innate responses are the ones that are already present

  • inside the cell that make a cell or a host,

  • you know, resistant to the virus.

  • Narrator: Unlike an adaptive response,

  • an innate one can attempt to fight off any pathogen

  • rather than a specific one.

  • Mucus membranes, stomach acids, skin, sentinel cells,

  • and even just a lack of the right receptors

  • can stop a virus from infecting a person.

  • So, this is when mutations are really important.

  • A successful spillover usually doesn't happen

  • with the original virus.

  • Parrish: They have to gain some mutations

  • that allow them to replicate most efficiently,

  • allow them to overcome those host barriers.

  • Narrator: A virus that infects

  • the digestive system of the pig

  • might attack respiratory cells in humans.

  • It depends on what receptor the virus is suited for

  • or mutates to be suited for.

  • Once inside the new host cell,

  • an infection will only be successful

  • if the virus can replicate.

  • Typically, infected cells will release interferons,

  • proteins that stop the virus from replicating

  • within the cell and in nearby cells,

  • which contain the infection and stop it

  • from spreading to new cells.

  • If that doesn't work,

  • the adaptive immune system kicks in.

  • Your T cells suss out and kill already infected cells

  • to stop them from making more new virus,

  • while your white blood cells pump out antibodies

  • specifically tailored to fight this new pathogen.

  • But because the body has never seen this virus before,

  • it can take weeks to produce the right ones.

  • And immunodeficiencies in either type of response

  • can make it even easier for a virus to take hold.

  • So, if a virus gets through all that,

  • contact, infection, replication,

  • then it has successfully spilled over.

  • But...

  • Parrish: The virus has to be able to transmit.

  • It has to be able to be shipped from that original person,

  • and it has to be able to infect, you know,

  • at least one or two more additional people

  • so that you can start a chain of transmission.

  • Narrator: A virus infecting two people

  • has double the odds of going on to infect additional people,

  • compared to a virus infecting just one.

  • And this can continue until it reaches

  • epidemic and pandemic proportions.

  • COVID-19 was certainly not the first zoonotic disease,

  • and it won't be the last.

  • Viruses don't want to kill their hosts;

  • no host means no virus.

  • But new diseases are so dangerous

  • because humans don't have the same immunity

  • as the virus' reservoir host.

  • And because there are so many,

  • it's currently not possible to predict

  • when or what specific viruses will spill over,

  • but we do know the conditions in which spillover can occur.

  • Parrish: You know, how and where they might occur

  • and how we can put in place sort of

  • better monitoring so that we can catch them early

  • and, as they say, stamp them out

  • before they get to the point where they become

  • sort of an out-of-control epidemic.

There are an estimated...

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B1 spillover infecting infect infection pathogen immune

How A Virus Like Coronavirus Jumps From Animals To People

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    林宜悉 posted on 2020/11/16
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