It starts with a tickle in your throat that becomes a cough.
你一开始是喉咙痒，而后转为咳嗽。
Your muscles begin to ache, you grow irritable, and you lose your appetite. It's official: you've got the flu.
肌肉开始疼痛，逐渐烦躁不安，并且丧失食欲。事实证明：你得了流行性感冒。
It's logical to assume that this miserable medley of symptoms is the result of the infection coursing through your body,
合理地推断，这些多种难受症状是流经你身体的感染所导致，
but is that really the case?
但真的是那样吗？
What's actually making you feel sick?
到底是什么让你觉得生病了？
What if your body itself was driving this vicious onslaught?
如果是身体自己不得不发起这猛烈的攻击呢？
You first get ill when a pathogen like the flu virus gets into your system, infecting and killing your cells.
发病初起，像流感病毒类的病原体侵入你的身体，感染并杀死细胞。
But this unwelcome intrusion has another effect: it alerts your body's immune system to your plight.
但这令人讨厌的入侵有另一效应：它让身体的免疫系统警觉到这个危险的状况。
As soon as it becomes aware of infection, your body leaps to your defense.
一旦它察觉到感染，身体会迅速启动防卫。
Cells called macrophages charge in as the first line of attack, searching for and destroying the viruses and infected cells.
巨噬细胞会当攻击先锋队冲向感染区，搜寻并杀死病毒及受感染的细胞。
Afterwards, the macrophages release protein molecules called cytokines
然后，巨噬细胞释放称为细胞因子的蛋白质分子，
whose job is to recruit and organize more virus-busting cells from your immune system.
它的任务是从免疫系统中动员并组织更多的搜捕病毒的细胞。
If this coordinated effort is strong enough, it'll wipe out the infection before you even notice it.
如果这协同作用力够强，它甚至在你觉得生病前就会歼灭这感染。
But that's just your body setting the scene for some real action.
但这只是身体为真正的战争做准备而已。
In some cases, viruses spread further, even into the blood and vital organs.
某些状况，病毒会进一步扩散，甚至侵入血液及重要器官。
To avoid this sometimes dangerous fate, your immune system must launch a stronger attack, coordinating its activity with the brain.
为避免这偶发的危险后果，免疫系统必须发动更强的反击，协同脑部一起展开行动。
That's where those unpleasant symptoms come in, starting with the surging temperature, aches and pains, and sleepiness.
那就是引起不适症状之所在开始时，体温剧升、肌肉酸痛及昏昏欲睡。
So why do we experience this?
为什么我们有这感受呢？
When the immune system is under serious attack, it secretes more cytokines, which trigger two responses.
当免疫系统遭受严重攻击，它会分泌更多细胞因子以激发两种反应。
First, the vagus nerve, which runs through the body into the brain,
第一，迷走神经是从全身汇聚至脑部
quickly transmits the information to the brain stem, passing near an important area of pain processing.
迅速地传递讯息到脑干，它会通过处理疼痛的重要区域附近。
Second, cytokines travel through the body to the hypothalamus,
第二，细胞因子从全身运行到下视丘
the part of the brain responsible for controlling temperature, thirst, hunger, and sleep, among other things.
这部分的脑负责控制体温、口渴、饥饿，及睡眠等等。
When it receives this message, the hypothalamus produces another molecule called prostaglandin E2, which gears it up for war.
当下视丘接收到讯息，它会制造另一种分子，称为“前列腺素E2”，让它进入备战。

The hypothalamus sends signals that instruct your muscles to contract and causes a rise in body temperature.
下视丘发出信号指示肌肉收缩，因而导致体温上升。
It also makes you sleepy, and you lose your appetite and thirst.
它也使你昏昏欲睡，以及丧失食欲和口渴感。
But what's the point of all of these unpleasant symptoms?
但所有这些不适症状有何意义呢？
Well, we're not yet sure, but some theorize that they aid in recovery.
这个我们还不确定，但有些推测它们会帮助复原。
The rise in temperature can slow bacteria and help your immune system destroy pathogens.
体温升高可阻碍细菌以及协助免疫系统杀死病原体。
Sleep lets your body channel more energy towards fighting infection.
睡眠让身体集中更多精力以对抗感染。
When you stop eating, your liver can take up much of the iron in your blood,
当停止进食，肝脏就能在血中吸收很多铁，
and since iron is essential for bacterial survival, that effectively starves them.
因为铁是细菌生存所必需的，如此可有效地使之匮乏。
Your reduced thirst makes you mildly dehydrated, diminishing transmission through sneezes, coughs, vomit, or diarrhea.
口渴感降低使身体呈现轻度脱水，减少经由打喷嚏、咳嗽、呕吐或腹泻的传播。
Though it's worth noting that if you don't drink enough water, that dehydration can become dangerous.
但值得注意的是，如果饮水不足可能变成危险的脱水。
Even the body's aches make you more sensitive, drawing attention to infected cuts that might be worsening, or even causing your condition.
连身体疼痛也会使你变得更敏感，让你注意到有个感染的伤口可能加重，或造成了你目前的状况。
In addition to physical symptoms, sickness can also make you irritable, sad, and confused.
除了身体症状外，生病也会令你感到烦躁、忧伤以及思想混乱。
That's because cytokines and prostaglandin can reach even higher structures in your brain,
那是因为细胞因子和前列腺素能到达脑部更高层的组织，
disrupting the activity of neurotransmitters, like glutamate, endorphins, serotonin, and dopamine.
干扰神经传导物质的运作，例如：谷氨酸、内啡肽、血清素及多巴胺。
This affects areas like the limbic system, which oversees emotions, and your cerebral cortex, which is involved in reasoning.
这会影响一些区域，如监督情绪的边缘系统及参与推理的大脑皮质。
So it's actually the body's own immune response that causes much of the discomfort you feel every time you get ill.
所以事实上是身体自己的免疫系统导致每次感冒时的诸多不舒服。
Unfortunately, it doesn't always work perfectly.
不幸的是，免疫系统并不总是运作完美。
Most notably, millions of people worldwide suffer from autoimmune diseases,
最值得一提的是，全球有数百万人罹患自体免疫性疾病，
in which the immune system treats normal bodily cues as threats, so the body attacks itself.
也就是免疫系统将正常身体信号视为威胁，因而攻击自己的身体。
But for the majority of the human race,
但对大部份的人类而言，
millions of years of evolution have fine-tuned the immune system so that it works for, rather than against us.
数百万年来的演化已将免疫系统精细调整以便为人类效力，而非对抗。
The symptoms of our illnesses are annoying, but collectively,
生病的症状虽很恼人，但总体而论，
they signify an ancient process that will continue barricading our bodies against the outside world for centuries to come.
它们象征着一种久远的过程，在未来的世纪将继续保卫我们的身体，对抗外在的世界。