How do viruses jump from animals to humans? - Ben Longdon

譯者: Ann Chen 審譯者: Bruce Sung

At a Maryland country fair in 2017,

2017 年，在馬里蘭州的郡市集，

the prize pigs were not looking their best.

參賽的豬隻看起來狀況欠佳。

Farmers reported feverish hogs with inflamed eyes and running snouts.

農夫通報發燒的豬隻 眼睛紅腫、鼻水直流。

But while fair officials worried about the pigs,

當市集官員擔心這些豬隻時，

the Maryland department of health was concerned about a group of sick fairgoers.

馬里蘭州衛生署正在留意 一群生病的參觀者。

Some had pet the pigs, while others had merely been near their barns;

其中有些人摸過豬隻， 有些只不過靠近豬圈而已。

but soon, 40 of these attendees would be diagnosed with swine flu.

很快地，當中的 40 個人 被診斷出得到豬流感。

More often than not, sick animals don’t infect humans.

通常，生病的動物不會感染人類，

But when they do, these cross-species infections,

但當感染人類時， 這些跨物種的感染──

or viral host jumps,

也就是病毒宿主轉跳──

have the potential to produce deadly epidemics.

有引發致命性流行病的可能。

So how can pathogens from one species infect another,

那麼病原體如何從一物種 感染另一物種，

and what makes host jumps so dangerous?

是什麼使得宿主轉跳如此危險呢？

Viruses are a type of organic parasite infecting nearly all forms of life.

病毒是一種有機寄生體， 幾乎可感染所有生命體。

To survive and reproduce, they must move through three stages:

為了存活及繁殖， 牠們必須歷經三個階段：

contact with a susceptible host, infection and replication,

接觸易受感染的宿主，感染與複製，

and transmission to other individuals.

以及傳播到其他個體。

As an example, let’s look at human influenza.

拿人類的流感為例，

First, the flu virus encounters a new host

首先，流感病毒接觸到新宿主，

and makes its way into their respiratory tract.

且進入他們的呼吸道，

This isn’t so difficult, but to survive in this new body,

這並不困難， 但要在新宿主體內存活，

the virus must mount a successful infection

牠必須發動成功的感染，

before it’s caught and broken down by an immune response.

且要在牠被宿主的免疫反應 逮到、分解之前完成。

To accomplish this task,

為達成目標，

viruses have evolved specific interactions with their host species.

病毒演化出與宿主間的 特殊交互作用。

Human flu viruses are covered in proteins

人類流感病毒上包覆著 經過演化的蛋白質，

adapted to bind with matching receptors on human respiratory cells.

能與人類呼吸道細胞的受體結合。

Once inside a cell, the virus employs additional adaptations

一旦進入細胞， 病毒會用其他已進化的構造

to hijack the host cell’s reproductive machinery

來操控宿主細胞的繁殖組織，

and replicate its own genetic material.

並複製牠自己的基因體。

Now the virus only needs to suppress or evade the host’s immune system

現在病毒只需要抑制 或避開宿主的免疫系統，

long enough to replicate to sufficient levels and infect more cells.

到足以複製夠多的數量 來感染更多細胞。

At this point, the flu can be passed on to its next victim

此時，流感病毒 就可藉由被感染的體液

via any transmission of infected bodily fluid.

傳播到下一位受害者。

However, this simple sneeze also brings the virus in contact with pets,

然而，僅是打噴嚏 也能將病毒傳到寵物、

plants, or even your lunch.

植物、甚至你的中餐上面。

Viruses are constantly encountering new species and attempting to infect them.

病毒不斷地接觸 並試圖感染新的物種，

More often than not, this ends in failure.

通常這會失敗，

In most cases, the genetic dissimilarity between the two hosts is too great.

大多因為宿主間的基因差異太大。

For a virus adapted to infect humans,

對演化來感染人類的病毒而言，

a lettuce cell would be a foreign and inhospitable landscape.

萵苣細胞是一個陌生 且不適合生存的環境。

But there are a staggering number of viruses circulating in the environment,

但在環境中流竄著數量繁多的病毒，

all with the potential to encounter new hosts.

牠們都有碰觸新宿主的可能。

And because viruses rapidly reproduce by the millions,

由於病毒快速地 以數百萬的數量複製，

they can quickly develop random mutations.

牠們能很快地發生隨機的突變。

Most mutations will have no effect, or even prove detrimental;

大部份的突變不會有用， 甚至對自己有害，

but a small proportion may enable the pathogen to better infect a new species.

但少部份可讓病原體 更容易感染新物種。

The odds of winning this destructive genetic lottery increase over time,

這種致病性基因的樂透勝率，

若時間夠久，或新物種與病毒舊宿主 有近親關係時會增加。

or if the new species is closely related to the virus’ usual host.

For a virus adapted to another mammal,

對已適應另一種哺乳類的病毒而言，

infecting a human might just take a few lucky mutations.

可能只需少許幸運的突變 就可感染人類，

And a virus adapted to chimpanzees,

而已經適應人類近親之一的 黑猩猩的病毒，

one of our closest genetic relatives, might barely require any changes at all.

可能幾乎不需任何的基因改變。

It takes more than time and genetic similarity

宿主轉跳要成功， 不只是靠時間及基因相似而已，

for a host jump to be successful.

Some viruses come equipped to easily infect a new host’s cells,

有些病毒雖與生俱有 容易感染新宿主的能力，

but are then unable to evade an immune response.

但卻無法逃避免疫系統的反應。

Others might have a difficult time transmitting to new hosts.

有些則是不易傳播給新宿主，

For example, they might make the host’s blood contagious,

例如，牠們可能使得 宿主的血液有感染性，

but not their saliva.

但唾液卻沒有。

However, once a host jump reaches the transmission stage,

然而，宿主轉跳 一旦進入傳播階段時，

the virus becomes much more dangerous.

病毒就變得更加危險。

Now gestating within two hosts,

在兩個宿主體內孕育時，

the pathogen has twice the odds of mutating into a more successful virus.

病原體有兩倍的機會 能突變為更成功的病毒，

And each new host increases the potential for a full-blown epidemic.

且每個新宿主都會增加 引發大流行的可能。

Virologists are constantly looking for mutations

病毒學家一直在尋找

that might make viruses such as influenza more likely to jump.

能讓病毒（如流感） 更容易轉跳的突變。

However, predicting the next potential epidemic is a major challenge.

然而，預測下次大流行是一大挑戰，

There’s a huge diversity of viruses that we’re only just beginning to uncover.

我們才開始陸續發現 這些多樣化的病毒而已。

Researchers are tirelessly studying the biology of these pathogens.

研究員孜孜不倦地 探究病原體的生物學，

And by monitoring populations to quickly identify new outbreaks,

藉由監測人群以便迅速地 發現流行病的爆發，

they can develop vaccines and containment protocols to stop these deadly diseases.

他們能研發疫苗及設計隔離計畫， 以便阻止這些致命性的疾病。