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'The flu' has become a slang term that refers to almost any ailment where you juuuust don't feel quite right.
如今，“流感”（flu）一词已经变成了一个俗语，几乎所有让你感觉不太舒服的疾病都能用它来形容。
But influenza proper is a handful of specific viruses that invade the respiratory lining, evade the immune system,
然而，“流行性感冒”（influenza）一词本身指的则是几种特定的病毒，它们不仅能侵入呼吸道，躲过免疫系统的检查，
and mutate at a rate that even the world's most brilliant doctors, scientists, and health organizations struggle to keep up with.
还能以世界上最优秀的医生、科学家和卫生组织都难以跟进的速度变异。
So what is Influenza exactly, and why does it always seem to be one step ahead of us?
那么问题来了，“流行性感冒”到底是个什么东西，为何它总能领先我们一步？
Some of the confusion around the term 'flu' may stem back to the word's roots: "Influenza" is an Italian word that just means, 'influence.'
“流感”一词之所以会让大家感到困惑，部分原因可能跟这个词的词根有关：“Influenza”本来是一个意大利词，其本意是“显灵”。
"Because, back 500 years ago, people didn't know about germs then.
“因为500年前，人们对细菌还不了解。
They thought if you got this disease, you were under the influence, perhaps of the stars or something else.
他们以为，如果你得了这种病，那可能是星星或其他什么东西在你身上显灵了。
My name is David Morens.
我叫大卫·莫伦斯。
I'm a medical doctor and an epidemiologist, and a virologist, and a historian, at the National Institutes of Health,
我是美国国立卫生研究院的一名医生/流行病学家/病毒学家/历史学家，
and an officer in the United States Public Health Service, and I'm here today to talk about one of my favorite subjects: influenza."
同时还在美国公共卫生署任职，今天，我就来跟大家聊聊我最喜欢的话题之一：流行性感冒。”
Influenza presents with such iconic symptoms that many other diseases are classified by their similarity to it.
流行性感冒的症状特别典型，所以很多疾病都是根据其与流行性感冒的症状的相似度来分门别类的。
Tell your doctor you're experiencing 'flu-like symptoms,' and they'll know exactly what you mean:
如果你跟你的医生说你出现了“流感一样的症状”，他们一下子就知道你什么意思了：
coughing, fever, head and muscle aches, fatigue, and well, just general "malaise"...
咳嗽，发烧，头痛，肌肉酸痛，疲劳，还有……反正就是“哪哪儿都不舒服”。
"…which kind of means just feeling really bad. Anybody who's ever had flu knows what it is."
“…简单来说，就是感觉很难受。任何得过流感的人都知道那是怎样一种感觉。”
But while many illnesses are similar to influenza, there's only one True Flu. Well, four, actually...
不过，尽管不少疾病的症状都跟流行性感冒差不多，但真正的流感只有一种。嗯，准确来说是四种……
And trying to put a finger on its countless variations and mutations is actually where influenza starts to get the better of us.
事实上，就在我们试图摸清它那些数都数不清的变异和突变种类的时候，这点恰好也是流感开始领先我们的地方。
"Depending on how you cut the cake, there are billions and trillions and zillions of different types of influenza.
“根据切蛋糕的方式的不同，流行性感冒的种类可以分出几十亿种、上万亿甚至上千亿种。
But to start, we categorize them into four categories: Influenza A, B, C, and D.
但我们可以先将它们分为四类：甲型、乙型、丙型和丁型流行性感冒。
The ones we're most interested in because they affect humans every year are A and B.
而我们最感兴趣的是甲型和乙型流行性感冒，因为这两种流感每年都会复发。
C also infects humans, but it's pretty much a minor disease that affects children,
丙型流行性感冒也会感染人类，但这种流感一般比较轻微，感染对象多为儿童，
and D is mostly an animal disease that people generally don't get, or at least they don't get symptoms from it."
丁型通常是动物流感，人一般不会感染这种疾病，至少不会出现有关症状。”
Influenza A, B, and C for the kiddos, start by entering your upper respiratory tract,
就大家伙儿而言，甲型，乙型和丙型流感都是从进入你的上呼吸道开始的，
either when you breathe contaminated air, or touch your nose and mouth after coming into contact with the virus.
无论你是呼吸了被污染的空气感染上的，还是接触病毒后又摸了鼻子或嘴巴感染上的。
"The only cells it really infects are what we call respiratory epithelial cells,
这几种流感真正感染的是我们所说的呼吸道上皮细胞，
a one-cell layer of cells that kind of protect the breathing tubes and the lungs, all the way down to the bottom, where we have air sacs.
这是一层单细胞的细胞层，它保护着我们的呼吸道和肺，一直到肺部末端的气囊。
Those lining cells are the ones that get infected.
这些内衬细胞就是流感病毒会感染的细胞。
Once they reach your respiratory lining, the viruses attach to a specific kind of cell receptor, called sialic acid.
一旦到达呼吸道的内衬细胞，病毒就会附着在一种名为“唾液酸”的特殊细胞受体上。
"If you could see them up close, they'd look like branches of trees.
“近距离观察的话，你会看到这种病毒长得就像树枝一样。
They stick out and they go in all different directions.
它们朝着各个方向延伸了出来。
They kind of capture things, and by binding to it, it can be dragged inside the cell, where it causes the infection."
它们可以捕捉到东西，通过与对方结合被吸入对方细胞内感染对方。”
The influenza virus then makes its way through your respiratory passages,
接着，流感病毒会穿过你的呼吸道，
in rare cases, reaching all the way to the bottom of the lungs, and leading to severe pneumonia.
极少数情况下，还会一直到达你的肺部末段，引发严重的肺炎。
All this suspicious activity would normally cause your body to launch an immune response.
通常，病毒细胞的所有这些可疑的活动会激活你体内的免疫系统。
And usually, it tries to.
通常，免疫系统也都会努力反击。
But this is complicated by the fact that the virus and the immune cells coming to the rescue are separated by that epithelial barrier.
然而，问题并没有这么简单，因为病毒和前来救援的免疫细胞被上皮细胞形成的那道屏障隔开了。
"That's one of the reasons it makes it hard to develop strong immunity to flu, and to raise strong immunity by using a flu vaccine."
“这就是为什么我们很难培养出强有力的反击流感的免疫系统，也很难通过接种流感疫苗来提高免疫力的一个原因。”
And as if that weren't enough, even if you are able to mount a defense of some sort against the virus,
仿佛这还不够严重，就算你有幸真的能对病毒进行某种防御，
influenza has already invented several thousand more versions of itself, before you can blink an eye.
感染你的流感也已经在你还来不及眨眼的工夫内分化出了几千个不同的版本。
"Influenza viruses are not stable things.
“流感病毒不具备稳定性。
They're always moving and changing, day-to-day, hour-to-hour, minute-to-minute.
它们一直在移动在变化，一天不落而且每时每刻都在移动在变化。
Mutation is all they can do.
变异是它们唯一的本事。
There is no state of their existence in human beings that is not constantly mutating."
它们在人类身上就没有哪个时候是没有处在不断变异状态的。”
This is because of a machinery in their RNA that causes consistent errors in gene replication.
这是因为它们的RNA中有一种会造成基因复制过程不断出错的机制。
But, unlike for humans, these 'errors' actually works to the virus's advantage.
只不过，与人类不同的是，这些“错误”对病毒本身其实是有利的。
"They're good errors that help them to survive.
“那些错误是帮助他们生存下来的正面错误。
Even in your own body, if you are infected with an influenza virus today, you're not going to be infected by one virus.
即便是在你自己的身体里，如果你今天感染了流感病毒，你感染的也不是一种病毒，
You're going to be infected by let us say 1,000 or 10,000 viruses."
而是成千上万种病毒。
And it's this rapid-fire mutation that makes whipping up a vaccine for this set of viruses a pretty tall order.
正是这种快速的变异使得研制出针对某一组病毒的疫苗变成了一项相当艰巨的任务。
Each year, researchers from the World Health Organization and around the world develop one vaccine for the Northern Hemisphere, and one for the Southern.
每年，世界卫生组织和世界各地的研究人员都会为北半球开发一种疫苗，为南半球开发一种疫苗。
These vaccines are released six months apart, to coincide with winter's 'flu season' in each area.
这些疫苗发布的时间相差六个月，以配合每个地区冬季出现的“流感季”。
Their recipe is slightly different each time, and can be tailored for specific types of patients, like the elderly.
疫苗的成分每次都会稍有不同，还能为特定类型的患者，比如老人，量身定制。
"The current vaccines have four viruses: two influenza A, and two influenza B.
“目前的疫苗含有四种病毒：两种甲型流感病毒和两种乙型流感病毒。
And it may or may not work.
打完之后可能会起作用，也可能不会起作用。
Because in the meantime, the viruses circulating around the world may have mutated to the point where your best guess vaccine is worthless.
因为在你接种的时候，在世界各地传播的病毒可能已经变异到你按照最准确的猜测配制的疫苗都已经变得毫无用处的地步了。
So, the scientists have to guess now, what vaccine they're going to need six months from now.
也就是说，科学家们现在要猜测的是，六个月后需要的是什么样的疫苗。
It's like looking into a crystal ball."
感觉就好像用水晶球占卜一样。”
And in addition to this seasonal cycle, every three decades or so, the stakes ramp up,
而且，除了这种季节性的复发流感，每隔30年左右，赌注还会进一步增加，
and the disease is dramatically more hostile, causing a pandemic.
届时，其恶劣程度要大得多，感染范围极大。
"The years we have a pandemic, as many as 50% or 60% of all people might be infected within a year.
“全球性大流感爆发的时候，一年之内就能感染所有人口的50%~60%。
That virus causing a pandemic can come at any time of year.
引起这种流感的病毒一年中的任何时候都可能出现。
Eventually, we don't call it a pandemic anymore - we call it a seasonal virus."
最后，我们将不再称其为全球性大流感了，会改称‘季节性病毒’。”
But all over the world, research continues on influenza, and the hope of a universal vaccine could be in sight.
不过，世界各地有关流感的研究仍在持续，研发出针对各种流感的通用疫苗的那天已经指日可待。
"A lot of scientists, including NIH and CDC, are working very hard on this.
“很多科学家，包括国家卫生研究院和疾控中心的科学家，都在非常努力地开展这方面的研究。
And every year, all over the world, thousands of influenza viruses are being taken from patients, and sequenced.
而且，每年都有成千上万的流感病毒被从世界各地的患者身上提取出来测序。
This gives an idea about what directions the evolution of this virus is going.
这样一来，我们对流感病毒的进化方向就能有一个大致的了解。
By learning how the body can successfully control influenza, we have a better idea how we can mimic that with a vaccine."
通过了解人体如何成功地控制流感，对于如何让疫苗模拟人体的免疫系统，我们已经有了更清晰的概念。
But in the meantime…"Don't walk, run and get your flu shot, every year."
不过，就算是这样（大家也要）…“少走路、多跑步，记得打疫苗，每年都要打噢。”
