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。
