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, of microscopic germs

但你沒感覺到的是成千上萬的微小細菌

that have covertly traveled through the air and onto your clothing, hands and face.

正透過空氣悄悄附著在你的衣服、手和臉上。

As gross as this scenario sounds,

雖然聽起來噁心，

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

但這的確是我們感染病菌的常見途徑。

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

細菌傳播往往就在不經意間。

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

任何我們接觸的表面都有細菌。

When we talk about germs,

而我們所說的細菌，

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

其實指的是許多不同種類的微生物，

including bacteria, fungi, protozoa and viruses.

包含細菌、真菌、原生動物和病毒。

But what our germs all have in common is the ability to interact with our bodies and change how we feel and function.

細菌的共同點是會和身體互動並改變我們感覺和身體機能。

Scientists who study infectious diseases have wondered for decades why it is that some of these germs are relatively harmless,

數十年來，傳染疾病專家始終疑惑為何有些細菌無害，

while others cause devastating effects and can sometimes be fatal.

有些卻會嚴重影響健康，造成死亡。

We still haven't solved the entire puzzle,

這仍是未解謎團，

but what we do know is that the harmfulness, or virulence, of a germ is a result of evolution.

但我們知道的是，有害或致命的細菌是演化而來的。

How can it be that the same evolutionary process can produce germs that cause very different levels of harm?

那麼，同一個進化過程怎麼會產生危害程度不同的細菌呢？

The answer starts to become clear if we think about a germ's mode of transmission,

想想細菌傳播的方式，答案就呼之欲出。

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

細菌會從一個宿主身上轉移到另一個宿主。

A common mode of transmission occurs through the air, like the sneeze you just witnessed,

空氣是最常見的傳染途徑，就如剛剛看到的噴嚏。

and one germ that uses this method is the rhinovirus,

鼻病毒就是其中一種透過空氣傳播的病毒。

which replicates in our upper airways, and is responsible for up to half of all common colds.

鼻病毒會於上呼吸道複製，而一半以上的普通感冒都是鼻病毒導致。

Now, imagine that after the sneeze,

想像一下，剛才的噴嚏後，

one of three hypothetical varieties of rhinovirus,

假設有三種鼻病毒，

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

先稱它們為，「太多」「太少」和「剛剛好」

has been lucky enough to land on you.

它們都附著在你身上。

These viruses are hardwired to replicate,

病毒本來就會自己複製。

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

但因基因差異，複製速率會不同。

"Too much" multiplies very often, making it very successful in the short run.

「太多」複製速率高，讓其在短時間內十分成功。

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

使宿主付出慘痛代價。

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

快速複製的病毒會嚴重危害健康，

making cold symptoms more severe.

使感冒症狀加劇惡化。

If you're too sick to leave your home,

若你病到無法離家，

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

病毒就沒有任何機會找新宿主。

And if the disease should kill you, the virus' own life cycle will end along with yours.

若你因此病逝，病毒的生命週期也隨之消逝。

"Too little," on the other hand, multiplies rarely and causes you little harm in the process.

另一方面，「太少」複製量不足，對健康影響不太大 。

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

雖然你能仍能健康出門，和潛在宿主接觸，

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

但因無症狀，你可能完全不會打噴涕。

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無法感染別人。

Meanwhile, "just right" has been replicating quickly enough to ensure that you're carrying sufficient amounts of the virus to spread,

而「剛剛好」複製得正好讓你帶著足夠的病毒，足以傳播。

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

又不會讓你臥床不起。

And in the end, it's the one that will be most successful at transmitting itself to new hosts and giving rise to the next generation.

最終，這種病毒會比其他兩種更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,

此理論最先發展於 1980 年代早期，

it predicts that germs will evolve to maximize their overall success by achieving a balance between replicating within a host,

此學說假設細菌藉由在宿主體內複製到剛好數量，將成功的可能發展到最大，

which causes virulence, and transmission to a new host.

造成新宿主感染。

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

此學說假設隨鼻病毒進化其病毒強度會衰弱，

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

因為鼻病毒需透過近距離傳染。

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

對鼻病毒而言，可到處走動的宿主最棒了。

and indeed, that is what we see.

此推論也正如我們所見。

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

雖然大多數人都會流鼻水、咳嗽、打噴嚏，

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

普通感冒通常不會太嚴重，最多一星期就好了。

It would be great if the story ended there,

如果病毒都那麼單純就好了，

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，此傳播途徑不需我們走動傳遞。

and may even benefit from harming us since a sick and immobile person is easier for mosquitoes to bite.

我們一病不起反而更好，因為臥病在床的病人最方便蚊子叮。

We would expect germs that depend less on host mobility,

較不依賴宿主移動性的病菌，

like those transmitted by insects, water or food,

像那些通過昆蟲、水或食物傳播的病毒，

to cause more severe symptoms.

通常會引起相對嚴重的症狀。

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

那麼，可以做些什麼來降低傳染病的危害呢？

Evolutionary biologist Dr. Paul Ewald has suggested that we can actually direct their evolution through simple disease-control methods.

演化生物學家保羅·由窩德博士建議，我們可以通過簡單的疾病控制方法來導正病毒進化。

By mosquito-proofing houses, establishing clean water systems,

透過防蚊屋、建立乾淨供水系統

or staying home when we get a cold,

或感冒時呆在家裡，

we can obstruct the transmission strategies of harmful germs while creating a greater dependence on host mobility.

我們可以阻斷有害病菌傳播，同時增加其對宿主活動性的依賴。

So, while traditional methods of trying to eradicate germs may only breed stronger ones in the long run,

雖然從長遠來看，試圖根除細菌的傳統方法可能會培育出更強壯的細菌，

this innovative approach of encouraging them to evolve milder forms could be a win-win situation.

但以這種方式鼓勵細菌進化出更溫和的傳播形式也能營造雙贏局面。

Well, for the most part.

好吧，也不全如此。