How do germs spread (and why do they make us sick)? - Yannay Khaikin and Nicole Mideo

細菌傳播途徑 (和怎麼讓我們生病) - Yannay Khaikin & Nicole Mideo

譯者: Hsuan Ling 審譯者: Adrienne Lin

The sun is shining.

陽光閃耀

The birds are singing.

鳥兒高歌

It looks like the start of another lovely day.

似乎又是美好一天的開始

You're walking happily in the park, when, "Ah-choo!"

你開心地走在公園

突然 "哈啾"

A passing stranger has expelled mucus and saliva from their mouth and nose.

一個路人噴出鼻涕和口水

You can feel the droplets of moisture land on your skin,

你感覺到皮膚上濕濕的

but what you can't feel are the thousands, or even millions,

但感覺不到成千上萬的微小細菌

of microscopic germs that have covertly traveled through the air

正經空氣悄悄的

and onto your clothing, hands and face.

附著在你的衣服 手和臉上

As gross as this scenario sounds,

這聽起來有點噁心

it's actually very common for our bodies to be exposed to disease-causing germs,

卻是身體感染致病細菌的常見途徑

and most of the time, it's not nearly as obvious.

常常不經意就被傳染到

Germs are found on almost every surface we come into contact with.

任何接觸物的表面都有細菌

When we talk about germs,

我們所說的細菌

we're actually referring to many different kinds of microscopic organisms,

其實指的是許多不同種的微生物

including bacteria, fungi,

包含細菌 真菌 原生動物 病毒

protozoa and viruses.

But what our germs all have in common is the ability to interact with our bodies

細菌的共同特點是會和身體互動

and change how we feel and function.

並改變感覺和身體機能

Scientists who study infectious diseases have wondered for decades

數十幾年來 傳染疾病專家始終疑惑

why it is that some of these germs are relatively harmless,

為什麼有的細菌無害

有的卻會嚴重摧毀健康 造成死亡

while others cause devastating effects and can sometimes be fatal.

We still haven't solved the entire puzzle,

我們還沒解開謎團

but what we do know is that the harmfulness, or virulence,

但我們知道有害或致命細菌

of a germ is a result of evolution.

是進化的結果

How can it be that the same evolutionary process

同樣的進化過程怎會產生

can produce germs that cause very different levels of harm?

不同有害程度的細菌呢

The answer starts to become clear

來看看細菌傳染方式就會有答案

if we think about a germ's mode of transmission,

which is the strategy it uses to get from one host to the next.

傳染是細菌從一個宿主

到另一個宿主的途徑

A common mode of transmission occurs through the air,

最常見的是經由空氣傳染

like the sneeze you just witnessed,

像你剛看到的打噴嚏

and one germ that uses this method is the rhinovirus,

鼻病毒就會利用這個方式

which replicates in our upper airways,

在你的上呼吸道複製

and is responsible for up to half of all common colds.

一半以上的普通感冒 都是鼻病毒造成

Now, imagine that after the sneeze,

讓我們想像 經過剛才的噴嚏後

one of three hypothetical varieties of rhinovirus,

三種可能的鼻病毒

let's call them "too much," "too little," and "just right,"

先稱這三種病毒為

"太多" "太少" 和 "剛剛好"

has been lucky enough to land on you.

假設其中一種夠"幸運"跑到你身上

These viruses are hardwired to replicate,

病毒天生會複製自己

but because of genetic differences, they will do so at different rates.

但因基因不同 複製速率不同

"Too much" multiplies very often,

"太多" 短時間內 就可大量成功複製病毒

making it very successful in the short run.

However, this success comes at a cost to you, the host.

但卻使宿主你付出慘痛代價

A quickly replicating virus can cause more damage to your body,

快速複製的病毒嚴重危害健康

making cold symptoms more severe.

使感冒症狀加劇惡化

If you're too sick to leave your home,

若你病到無法起床

you don't give the virus any opportunities to jump to a new host.

病毒沒有任何機會找新宿主

And if the disease should kill you,

若你因此病逝

the virus' own life cycle will end along with yours.

病毒的生命週期也隨你消逝

"Too little," on the other hand, multiplies rarely

另一方面 "太少"複製量不大

and causes you little harm in the process.

因此對健康影響不太大

Although this leaves you healthy enough to interact with other potential hosts,

雖然你能仍健康出門 和潛在宿主接觸

the lack of symptoms means you may not sneeze at all,

但因無症狀 你完全不會打噴涕

or if you do, there may be too few viruses in your mucus to infect anyone else.

即使會 鼻涕裡少數的病毒 也不會影響別人

Meanwhile, "just right" has been replicating quickly enough

"剛剛好"複製的數量快又夠

to ensure that you're carrying sufficient amounts of the virus to spread

讓你帶著足夠的病毒傳播

but not so often that you're too sick to get out of bed.

但不會讓你病到臥床不起

And in the end, it's the one that will be most successful

這種病毒比其他兩種

at transmitting itself to new hosts and giving rise to the next generation.

最容易找到新宿主並繁衍後代

This describes what scientists call trade-off hypothesis.

這是科學家所謂的權衡理論

First developed in the early 1980s,

最先發展於1980年代早期

it predicts that germs will evolve to maximize their overall success

此學說假設細菌藉由

在宿主體內複製到數量剛好

by achieving a balance between replicating within a host,

發展最大成功的可能性

which causes virulence, and transmission to a new host.

造成新宿主感染病毒

In the case of the rhinovirus,

以鼻病毒的例子來說

the hypothesis predicts that its evolution will favor less virulent forms

此學說假設鼻病毒的進化會減弱病毒

because it relies on close contact to get to its next victim.

因為需透過近距離傳染

For the rhinovirus, a mobile host is a good host,

對鼻病毒而言 可到處走的宿主最棒

and indeed, that is what we see.

事實上 這也正是我們所見的

While most people experience a runny nose, coughing and sneezing,

大多數人流鼻水 感冒 打噴嚏

the common cold is generally mild and only lasts about a week.

普通感冒不會太嚴重 最多一星期就好

It would be great if the story ended there,

如果事情都那麼簡單就好了

but germs use many other modes of transmission.

但細菌會利用許多其他方法傳播

For example, the malaria parasite, plasmodium, is transmitted by mosquitoes.

例如 瘧疾寄生蟲 瘧原蟲 會經蚊子傳播

Unlike the rhinovirus, it doesn't need us to be up and about,

不像鼻病毒 不需我們站立走動

and may even benefit from harming us

並會從傷害我們中獲益

since a sick and immobile person is easier for mosquitoes to bite.

而臥病在床的病人最容易遭蚊子叮

We would expect germs that depend less on host mobility,

就如昆蟲 水和食物的細菌感染

like those transmitted by insects, water or food,

可假設這些細菌較不需宿主和他人接觸

to cause more severe symptoms.

來造成更嚴重症狀

So, what can we do to reduce the harmfulness of infectious diseases?

我們可以怎麼做降低感染疾病症狀

Evolutionary biologist Dr. Paul Ewald

生物進化學家保羅由窩德建議

has suggested that we can actually direct their evolution

可藉由簡單的疾病控制法主導細菌進化

through simple disease-control methods.

By mosquito-proofing houses, establishing clean water systems,

如防蚊屋 乾淨水源系統建立

or staying home when we get a cold,

或生病時待在家

we can obstruct the transmission strategies of harmful germs

我們可在身體製造

更多接觸傳染的細菌同時

while creating a greater dependence on host mobility.

斷絕有害細菌傳染途徑

So, while traditional methods of trying to eradicate germs

傳統預防細菌的方法

may only breed stronger ones in the long run,

長久下來可能造成更多強大細菌

this innovative approach of encouraging them to evolve milder forms

新的方法可使細菌進化較弱

could be a win-win situation.

這或許是雙贏方法

(Cough)

咳咳

Well, for the most part.

嗯 至少大部分時候是如此