Why is it so easy for birds to spread illness? - Benjamin Anderson · TED-Ed
了解禽流感病毒H5N1如何感染多种哺乳动物，以及它为何具有传播到人类的潜力。适合练习科学类词汇和理解病毒传播机制。
In March 2024, US veterinarians were facing down a mystery.
2024年3月，美国的兽医们面临一个尚未解开的谜团。
On dairy farms in Texas and nearby states, cows were losing their appetite and producing thick, discolored milk.
在德克萨斯州及邻近州的奶牛场中，奶牛的食欲明显下降，同时产出的牛奶质地黏稠且颜色异常。
At the same time, cats on these farms were being diagnosed with an unknown neurological disease.
与此同时，这些农场里的猫被诊断出患有某种未知的神经系统疾病。
For weeks, all their tests for common cattle diseases came back negative until lab results finally uncovered the culprit:
几周以来，他们对牛常见疾病的检测结果均为阴性；直到实验室最终发现了问题的根源：
influenza A subtype H5N1 — more commonly known as bird flu.
甲型流感H5N1亚型，通常被称为“禽流感”。
This wasn’t the first time a virus adapted for birds infected non-avian animals.
这并不是第一次出现针对鸟类的病毒感染非鸟类动物的情况。
In fact, every human influenza pandemic on record has been linked to some strain of bird flu.
事实上，有记录以来的每一次人类流感大流行都与某种禽流感病毒株有关。
So what is it about these viruses that makes them so frighteningly flexible?
那么，究竟是这些病毒的哪些特性使它们具备了如此惊人的“灵活性”（即能够轻易适应各种环境或变化）呢？
Of the four groups of flu viruses, all bird flus fall into group A.
在四种流感病毒类型中，所有禽流感病毒都属于A型。
Within this group, the viruses are organized into subtypes defined by two viral proteins.
在这个病毒群体中，这些病毒被划分为不同的亚型，而这些亚型的划分依据是两种特定的病毒蛋白。
Hemagglutinin binds to host cell receptors, allowing the virus to enter the cell and replicate.
血凝素能够与宿主细胞的受体结合，从而使病毒进入细胞并开始复制。
And neuraminidase helps those new viruses leave the cell.
神经氨酸酶帮助这些新病毒离开细胞。
Currently, we’ve discovered 18 types of hemagglutinin and 11 types of neuraminidase, and different subtypes of influenza A use different combinations.
目前，我们已经发现了 18 种血凝素和 11 种神经氨酸酶；不同亚型的甲型流感病毒会使用不同的这些物质的组合。
For example, the combo of hemagglutinin 5 and neuraminidase 1 is called H5N1.
例如，血凝素5（Hemagglutinin 5）与神经氨酸酶1（Neuraminidase 1）的组合被称为H5N1。
These proteins are necessary for influenza A to enter and exit host cells, so they generally determine which animals a strain can infect.
这些蛋白质对于甲型流感病毒进入和离开宿主细胞至关重要，因此它们通常决定了某种流感病毒株能够感染哪些动物。
But birds, especially wild aquatic birds, have receptors for most known versions of these proteins, making them susceptible to nearly all combinations.
然而，鸟类（尤其是野生水鸟）体内存在能够识别这些蛋白质几乎所有已知类型的受体，因此它们很容易受到各种病毒组合的感染。
Worse still, these waterfowl shed the virus through their feces
更糟糕的是，这些水禽会通过粪便排出病毒。
into lakes and ponds, increasing the odds of infection for any bird using those bodies of water.
这些污染物会进入湖泊和池塘中，从而增加任何使用这些水域的鸟类感染疾病的风险。
And since the viruses don’t usually make these birds actively sick, they can act as reservoirs for disease— flying around and spreading their strain.
由于这些病毒通常不会导致这些鸟类出现明显的病症，因此它们可以成为疾病的传播媒介：在空气中飞来飞去，将病毒传播给其他鸟类。
Thankfully, humans are mostly safe from all these flus.
幸运的是，人类大多不会受到这些流感病毒的威胁。
We only have receptors for a few viral proteins, protecting us from most combinations.
我们体内只有针对少数病毒蛋白的受体，这些受体能够保护我们免受绝大多数病毒组合的侵害。
And while bird flus can sometimes bind to receptors in our airways, our more vulnerable receptors are deep in our lungs.
虽然禽流感病毒有时会附着在我们呼吸道上的受体上，但真正容易受到感染的受体其实位于我们的肺部深处。
This makes infection less likely and transmission less efficient compared to common human flus that target receptors in our nose and throat.
与那些针对我们鼻腔和咽喉中受体的普通人类流感病毒相比，这种病毒使得感染的可能性更低，传播效率也更低。
Unfortunately, influenza has other tricks up its sleeves.
不幸的是，流感还有其他“杀手锏”（即其他致病方式）。
Viruses have antigens— specific molecules that our immune system can recognize and attack to fight against an infection.
病毒含有抗原，这些抗原是特定的分子，我们的免疫系统能够识别并攻击它们，从而抵御病毒感染。
But two processes are constantly changing these molecules and challenging our immune systems.
然而，有两种过程在不断改变这些分子，并对我们的免疫系统构成挑战。
The first is antigenic drift, where antigens change slowly over time due to random mutations.
第一种现象是“抗原漂变”（antigenic drift）：由于随机突变，抗原会随着时间的推移逐渐发生改变。
This makes it harder for immune systems to recognize the virus, but the process is relatively slow and trackable.
这使得免疫系统更难以识别这种病毒，不过这一过程相对较慢且可以被追踪。
In fact, we manage the antigenic drift of human flu by updating our flu shots every year.
事实上，我们通过每年更新流感疫苗来应对人类流感的抗原漂移现象。
The much more sudden and potentially dangerous process is antigenic shift.
更为突然且可能更具危险性的过程是“抗原性转变”（antigenic shift）。
This happens when two or more different influenza viruses meet inside a host and swap entire gene segments,
当两种或更多不同的流感病毒在宿主体内相遇时，它们会交换整个基因片段，从而引发这种情况。
instantly creating a new virus with all-new abilities— including the potential to bind to different receptors and infect different species.
这种病毒会立即生成一种全新的变种，具备全新的特性：例如能够与不同的受体结合，并感染不同的物种。
This is why bird flus are so concerning.
这就是为什么禽流感如此令人担忧的原因。
Since birds are vulnerable to so many influenza A subtypes, they’re natural reservoirs for antigenic shifts.
由于鸟类容易感染多种甲型流感病毒亚型，因此它们自然而然地成为了抗原变异的宿主（即这些病毒抗原变化的储存库）。
Consider our dairy farm mystery.
让我们来思考一下这个关于我们奶牛场的谜团吧。
While H5N1 used to infect mostly birds, in the past few years, it’s been found in over 70 mammal species,
虽然 H5N1 病毒最初主要感染鸟类，但在过去几年中，人们已经在 70 多种哺乳动物身上发现了该病毒的踪迹。
including cats, dairy cows, foxes, seals, and bears.
其中包括猫、奶牛、狐狸、海豹和熊。
Each new mammalian infection gives the virus more chances to drift or shift, potentially spilling over into humans,
每一起新的哺乳动物感染事件都为病毒提供了更多的变异或进化机会，这些病毒有可能随后传播给人类。
and igniting a pandemic.
从而引发了一场全球性的大流行病。
And since our immune systems aren’t trained to recognize avian influenza proteins, the effects of these viruses can be deadly.
由于我们的免疫系统并未被训练来识别禽流感的蛋白质，这些病毒可能会带来致命后果。