this many individual viruses on Earth.

地球上這麼多的病毒個體。

Luckily, there are only a little over

幸運的是，只有一點多

1,000 virus strains known to infect humans,

已知感染人類的1000種病毒株。

the majority of which come from other animals.

其中大部分來自其他動物。

They're part of a group called zoonotic diseases,

它們是人畜共患疾病的一部分。

caused by bacteria, viruses, parasites, or fungi.

由細菌、病毒、寄生蟲或真菌引起；

And the one we're all focusing on now

而我們現在都在關注的是

is the virus that causes COVID-19.

是導致COVID-19的病毒。

And new ones can emerge at any time.

而新的隨時可能出現。

Here's what needs to happen

下面是需要發生的事情

for a virus to jump from animals to humans.

讓病毒從動物跳到人類身上。

As a pathogen, the virus' goal

作為一種病原體，該病毒的目標是

is to infect its host and replicate,

是為了感染宿主並進行復制。

because it can't do that on its own.

因為它自己無法做到這一點。

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

就說這頭豬是病毒的原宿主。

He and his buddies form a reservoir,

他和他的夥伴們組成了一個水庫。

a specific population of animals of the same species

獸類

that naturally host a pathogen.

自然寄生病原體的。

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

科林・帕裡什。所以，有數以百萬計的病毒有

infecting animals. Literally millions.

感染動物。從字面上看，數以百萬計。

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

我的意思是，我們看的越多，發現的越多。

And most of those viruses

而這些病毒中的大部分

don't infect, you know, other animals.

不要感染，你知道，其他動物。

They have a restricted host range.

它們的主機範圍有限。

Narrator: Most of the time, the virus

旁白：大多數時候，病毒

doesn't affect its original host

不影響原主機

or only mildly affects it.

或只是輕度影響。

So, what's it doing in there?

那麼，它在裡面做什麼？

Suppose that this pig hosts a virus

假設這頭豬攜帶了一種病毒

that primarily infects the gut.

主要感染腸道的。

Viruses tend to attack different parts of the body,

病毒往往攻擊身體的不同部位。

depending on whether they can bind to these guys.

就看他們能不能和這些人綁定了。

Receptors are proteins found along the outside of a cell,

受體是沿著細胞外部發現的蛋白質。

used to communicate with the rest of the body.

用來與身體其他部位進行交流。

But viruses can latch on, too.

但是，病毒也可以吸附在上面。

Cells in a respiratory system may have different receptors

呼吸系統中的細胞可能有不同的受體。

than cells in a digestive tract.

比消化道的細胞。

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

病毒是通過吸附在宿主細胞上進行活動的。

entering it or injecting bits of itself into it,

進入其中或將自己的部分注入其中。

and then hijacking it.

然後劫持它。

It forces the cell to make copies of the virus,

它迫使細胞製造病毒的副本。

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

都會繼續劫持其他宿主細胞。

This will usually kill the host cell.

這通常會殺死宿主細胞。

And if enough of the host cells are infected

如果有足夠多的宿主細胞被感染的話

and make more of the virus,

並使更多的病毒。

the host will contract an infection,

宿主會受到感染。

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

其中，如果身體無法抗衡或抗衡過猛。

could lead to severe disease or death.

可能導致嚴重疾病或死亡。

In reservoirs, however, the species has likely evolved

然而，在水庫中，該物種很可能已經演化出了

a resistance to the virus over many generations.

在許多代人中產生對病毒的抵抗力。

This allows a sort of equilibrium;

這樣就可以達到一種平衡。

the immune system controls the infection

免疫系統控制感染

without killing the virus off completely.

而沒有將病毒徹底殺滅。

If the virus jumps, though,

如果病毒跳動，但。

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

新的主機不會有同樣的，或任何免疫力。

That might sound scary, because truthfully

這可能聽起來很可怕，因為事實上

you are constantly being exposed to viruses.

你不斷地被暴露在病毒中。

But only a very small number

但只有極少數人

succeed at infecting a new host species.

成功感染新的宿主物種。

It's called spillover,

這就是所謂的溢出效應。

and there are a series of barriers

並且有一系列的障礙

that a virus must navigate for that to happen.

病毒必須通過導航才能實現。

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

如果它被哪怕是其中的一個撐死了。

it can be stopped in its tracks.

它可以被阻止在其軌道上。

Simplified, they represent two things:

簡而言之，它們代表了兩件事。

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

病毒能否進入新宿主的細胞。

and can it bind and enter them?

並能束縛和進入他們嗎？

The more infected pigs there are in one space

在一個空間裡，感染的豬越多。

and the closer they are to people,

且與人的關係越密切。

generally the more chance of spillover.

一般來說，溢出的機會越多。

But the likelihood also has to do with

但這種可能性還與

how the humans are interacting with them.

人類是如何與它們互動的。

Animal viruses are usually transmitted to people

動物病毒通常會傳染給人

in a few ways: contact with excretions,

在幾個方面：與排洩物接觸。

slaughter, bites, contact with tissues,

屠宰、咬傷、接觸組織；

or through an intermediate species

或通過中間種

like mosquitoes or ticks.

如蚊子或蜱蟲。

So places like farms and slaughterhouses

所以，像農場和屠宰場這樣的地方。

and even petting zoos,

甚至是寵物樂園。

where people come in close contact with animals,

人們與動物親密接觸的地方。

have an increased risk of spillover.

有增加外溢的風險。

Proximity alone isn't enough, though.

不過，光是近距離還不夠。

Some of it can be genetic for humans.

有些可能是人類的遺傳。

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

有一個龐大的基因列表，已經相關的

with different risks of infections,

具有不同的感染風險。

some genes offering resistance to certain infections

一些基因提供對某些感染的抵抗力

and others increasing risk.

和其他人增加風險。

Beyond genes, the virus has to get through

除了基因之外，病毒還要通過

the body's innate immune responses.

身體的先天免疫反應。

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

帕裡什所以，有兩種類型的免疫反應。

One is the simple adaptive immune responses,

一是簡單的適應性免疫反應。

so those are antibodies and T cells,

所以這些都是抗體和T細胞。

and they generally get stimulated

他們一般會受到刺激

after the infection has already occurred.

感染已經發生後。

The innate responses are the ones that are already present

先天的反應是已經存在的反應。

inside the cell that make a cell or a host,

細胞內，使細胞或宿主。

you know, resistant to the virus.

你知道，抗病毒。

Narrator: Unlike an adaptive response,

旁白：不像適應性反應

an innate one can attempt to fight off any pathogen

先天的，可以嘗試對抗任何病原體。

rather than a specific one.

而不是一個具體的。

Mucus membranes, stomach acids, skin, sentinel cells,

粘膜、胃酸、皮膚、哨兵細胞。

and even just a lack of the right receptors

甚至只是缺乏正確的受體。

can stop a virus from infecting a person.

可以阻止病毒感染人。

So, this is when mutations are really important.

所以，這時候突變才是真正重要的。

A successful spillover usually doesn't happen

成功的外溢通常不會發生。

with the original virus.

與原始病毒。

Parrish: They have to gain some mutations

帕裡什。他們必須獲得一些突變

that allow them to replicate most efficiently,

使它們能夠最有效地複製。

allow them to overcome those host barriers.

使他們能夠克服這些東道主的障礙。

Narrator: A virus that infects

旁白：感染的病毒

the digestive system of the pig

豬的消化系統

might attack respiratory cells in humans.

可能會攻擊人類的呼吸道細胞。

It depends on what receptor the virus is suited for

這要看病毒適合什麼受體了

or mutates to be suited for.

或變異為適合。

Once inside the new host cell,

一旦進入新的宿主細胞內。

an infection will only be successful

感染才會成功

if the virus can replicate.

如果病毒能夠複製。

Typically, infected cells will release interferons,

通常情況下，受感染的細胞會釋放干擾素。

proteins that stop the virus from replicating

阻止病毒複製的蛋白質

within the cell and in nearby cells,

在細胞內和附近的細胞中。

which contain the infection and stop it

感染，並阻止它

from spreading to new cells.

從傳播到新的細胞。

If that doesn't work,

如果這還不行

the adaptive immune system kicks in.

適應性免疫系統啟動。

Your T cells suss out and kill already infected cells

你的T細胞發現並殺死已經感染的細胞。

to stop them from making more new virus,

以阻止他們製造更多的新病毒。

while your white blood cells pump out antibodies

而你的白細胞則會泵出抗體。

specifically tailored to fight this new pathogen.

專門為對抗這種新的病原體而設計。

But because the body has never seen this virus before,

但因為身體從來沒有見過這種病毒。

it can take weeks to produce the right ones.

它可能需要幾個星期才能生產出合適的。

And immunodeficiencies in either type of response

而任何一種反應類型的免疫缺陷

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

可以讓病毒更容易得手。

So, if a virus gets through all that,

所以，如果一個病毒通過所有這些。

contact, infection, replication,

接觸、感染、複製。

then it has successfully spilled over.

那麼它已經成功溢出。

But...

但是...

Parrish: The virus has to be able to transmit.

帕裡什。病毒必須能夠傳播。

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

它必須能從那個原人運來。

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

它必須能夠感染，你知道的。

at least one or two more additional people

至少再增加一兩個人

so that you can start a chain of transmission.

以便你可以開始連鎖傳輸。

Narrator: A virus infecting two people

旁白：一種病毒感染了兩個人

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

有雙倍的機率去感染更多的人。

compared to a virus infecting just one.

相比於病毒只感染一個。

And this can continue until it reaches

而這種情況可以持續到它達到

epidemic and pandemic proportions.

流行病和大流行病的規模；

COVID-19 was certainly not the first zoonotic disease,

COVID-19當然不是第一個人畜共患病。

and it won't be the last.

而且這不會是最後一次。

Viruses don't want to kill their hosts;

病毒不想殺死它們的宿主。

no host means no virus.

沒有主機就沒有病毒。

But new diseases are so dangerous

但新的疾病是如此危險

because humans don't have the same immunity

因為人類沒有同樣的免疫力。

as the virus' reservoir host.

作為病毒的宿主。

And because there are so many,

因為有這麼多。

it's currently not possible to predict

目前無法預測

when or what specific viruses will spill over,

什麼時候或什麼特定的病毒會溢出來。

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

但我們確實知道在什麼情況下會發生溢出效應。

Parrish: You know, how and where they might occur

帕裡什。你知道，如何以及在哪裡發生

and how we can put in place sort of

以及我們如何能把在某種程度上

better monitoring so that we can catch them early

更好的監測，以便我們能夠及早發現它們

and, as they say, stamp them out

趕盡殺絕

before they get to the point where they become

在他們變得

sort of an out-of-control epidemic.

某種失控的流行病。