Your muscles begin to ache,

肌肉開始疼痛

you grow irritable,

逐漸煩躁不安

and you lose your appetite.

並且喪失食慾

It's official: you've got the flu.

事實証明：你得了流行性感冒！

It's logical to assume that this miserable medley of symptoms

合理地推斷，這些多種難受症狀

is the result of the infection coursing through your body,

是流經你身體的感染所導致

but is that really the case?

但真的是那樣嗎？

What's actually making you feel sick?

到底是什麼讓你覺得生病了？

What if your body itself was driving this vicious onslaught?

如果是身體自己 不得不發起這猛烈的攻擊呢？

You first get ill when a pathogen like the flu virus gets into your system,

發病初起，像流感病毒類的病原體 侵入你的身體

infecting and killing your cells.

感染並殺死細胞

But this unwelcome intrusion has another effect:

但這令人討厭的入侵有另一效應：

it alerts your body's immune system to your plight.

它讓身體的免疫系統 警覺到這危險狀況

As soon as it becomes aware of infection, your body leaps to your defense.

一旦它察覺到感染 身體會迅速啟動防衛

Cells called macrophages charge in as the first line of attack,

巨噬細胞會當攻擊先鋒隊 衝向感染區

searching for and destroying the viruses and infected cells.

搜尋並殺死病毒及受感染的細胞

Afterwards, the macrophages release protein molecules called cytokines

然後，巨噬細胞釋放 稱為 細胞因子 (cytokines) 的蛋白質分子

whose job is to recruit

它的任務是從免疫系統中 動員並組織更多的搜捕病毒的細胞

and organize more virus-busting cells from your immune system.

如果這協同作用力夠強

If this coordinated effort is strong enough,

它甚至在你覺得生病前 就會殲滅這感染

it'll wipe out the infection before you even notice it.

但這只是身體 為真正的戰爭做準備而已

But that's just your body setting the scene for some real action.

某些狀況，病毒會進一步擴散

In some cases, viruses spread further,

甚至侵入血液及重要器官

even into the blood and vital organs.

為避免這偶發的危險後果

To avoid this sometimes dangerous fate,

免疫系統必須發動更強的反擊

your immune system must launch a stronger attack,

協同腦部一起展開行動

coordinating its activity with the brain.

那就是引起不適症狀之所在

That's where those unpleasant symptoms come in,

開始時，體溫劇升

starting with the surging temperature,

肌肉酸痛

aches and pains,

及昏昏欲睡

and sleepiness.

為什麼我們有這感受呢？

So why do we experience this?

當免疫系統遭受嚴重攻擊

When the immune system is under serious attack,

它會分泌更多細胞因子

it secretes more cytokines,

以激發兩種反應

which trigger two responses.

第一，迷走神經是從全身 匯聚至腦部

First, the vagus nerve, which runs through the body into the brain,

迅速地傳遞訊息到腦幹

quickly transmits the information to the brain stem,

它會通過處理疼痛的重要區域附近

passing near an important area of pain processing.

第二，細胞因子從全身運行到下視丘

Second, cytokines travel through the body to the hypothalamus,

這部份的腦是負責控制體溫、

the part of the brain responsible for controlling temperature,

口渴、飢餓，及睡眠等等

thirst,

當下視丘接收到訊息

hunger,

它會製造另一種分子

and sleep,

稱為「前列腺素E2」 讓它進入備戰

among other things.

下視丘發出信號指示肌肉收縮

When it receives this message,

因而導致體溫上升

the hypothalamus produces another molecule

它也使你昏昏欲睡

called prostaglandin E2, which gears it up for war.

以及喪失食慾和口渴感

The hypothalamus sends signals that instruct your muscles to contract

但所有這些不適症狀 有何意義呢？

and causes a rise in body temperature.

這個…. 我們還不確定

It also makes you sleepy,

但有些推測它們會幫助復原

and you lose your appetite and thirst.

體溫升高可阻礙細菌

But what's the point of all of these unpleasant symptoms?

以及協助免疫系統殺死病原體

Well, we're not yet sure,

睡眠讓身體集中更多精力 以對抗感染

but some theorize that they aid in recovery.

當停止進食， 肝臟就能在血中吸收很多鐵

The rise in temperature can slow bacteria

因為鐵是細菌生存所必需

and help your immune system destroy pathogens.

如此可有效地使之匱乏

Sleep lets your body channel more energy towards fighting infection.

口渴感降低 使身體呈現輕度脫水

When you stop eating, your liver can take up much of the iron in your blood,

減少經由打噴嚏、 咳嗽、嘔吐或腹瀉的傳播

and since iron is essential for bacterial survival,

但值得注意的是如果飲水不足

that effectively starves them.

可能變成危險的脫水

Your reduced thirst makes you mildly dehydrated,

連身體疼痛也會使你變得更敏感

diminishing transmission through sneezes,

讓你注意到有個感染的傷口 可能加重

coughs,

或造成你目前的狀況

vomit,

除了身體症狀外

or diarrhea.

生病也會令你感到煩躁、 憂傷以及思想混亂

Though it's worth noting that if you don't drink enough water,

那是因為細胞因子和前列腺素

that dehydration can become dangerous.

能到達腦部更高層的組織

Even the body's aches make you more sensitive,

干擾神經傳導物質的運作

drawing attention to infected cuts that might be worsening,

例如：穀胺酸 (glutamate)、

or even causing your condition.

內啡肽 (endorphin)、

In addition to physical symptoms,

血清素 (serotonin)

sickness can also make you irritable,

及多巴胺 (dopamine)

sad,

這會影響一些區域，如 監督情緒的邊緣系統（Limbic System）

and confused.

及參與推理的大腦皮質

That's because cytokines and prostaglandin

所以事實上是身體自己的免疫系統

can reach even higher structures in your brain,

導致每次感冒時的諸多不舒服

disrupting the activity of neurotransmitters,

不幸的是，免疫系統並不總是運作完美

like glutamate,

最值得一提的是全球有數百萬人 罹患「自體免疫性疾病」

endorphins,

也就是免疫系統 將正常身體信號視為威脅

serotonin,

因而攻擊自己的身體

and dopamine.

但對大部份的人類而言

This affects areas like the limbic system, which oversees emotions,

數百萬年來的演化 已將免疫系統精細調整

and your cerebral cortex, which is involved in reasoning.

以便為人類效力，而非對抗

So it's actually the body's own immune response

生病的症狀雖很惱人

that causes much of the discomfort you feel every time you get ill.

但總體而論，它們象徵著一種久遠的過程

Unfortunately, it doesn't always work perfectly.

在未來的世紀，將繼續保衛我們的身體 對抗外在的世界

Most notably, millions of people worldwide suffer from autoimmune diseases,

翻譯：Ann Chen

in which the immune system treats normal bodily cues as threats,

so the body attacks itself.

But for the majority of the human race,

millions of years of evolution have fine-tuned the immune system

so that it works for, rather than against us.

The symptoms of our illnesses are annoying,

but collectively, they signify an ancient process

that will continue barricading our bodies against the outside world

for centuries to come.