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.