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At a Maryland country fair in 2017, the prize pigs were not looking their best.
在马里兰州2017年的郡博览会上，获奖的猪看起来状态并不好。
Farmers reported feverish hogs with inflamed eyes and running snouts.
农民报告说，这些发烧的猪眼睛红肿、鼻涕直流。
But while fair officials worried about the pigs, the Maryland department of health was concerned about a group of sick fairgoers.
不过，虽然展会官员担心这些猪，但马里兰州卫生部门则对一组生病的参会者很重视。
Some had pet the pigs, while others had merely been near their barns; but soon, 40 of these attendees would be diagnosed with swine flu.
有些人摸过这些猪，而有些人仅从它们的圈旁路过；但很快，这些参与者中40人将被诊断患猪流感。
More often than not, sick animals don't infect humans.
生病动物通常不会感染人类。
But when they do, these cross-species infections, or viral host jumps, have the potential to produce deadly epidemics.
但是当它们传染时，这些跨物种感染，也叫病毒宿主转移，可能产生致命的流行病。
So how can pathogens from one species infect another, and what makes host jumps so dangerous?
那么一个物种的病原体如何感染另一个物种的，什么使宿主转移如此危险呢？
Viruses are a type of organic parasite infecting nearly all forms of life.
病毒是一种感染几乎所有生命形式的有机寄生物。
To survive and reproduce, they must move through three stages:
为了生存和繁殖，它们必须经历三个阶段：
contact with a susceptible host, infection and replication, and transmission to other individuals.
与易感宿主接触、感染和复制、以及传递给其他人。
As an example, let's look at human influenza.
举个例子，我们来看看人类流感。
First, the flu virus encounters a new host and makes its way into their respiratory tract.
首先，流感病毒遇到了新的宿主，并进入他们的呼吸道。
This isn't so difficult, but to survive in this new body,
进入并不困难，但要在这个新的身体中生存，
the virus must mount a successful infection before it's caught and broken down by an immune response.
在病毒被免疫反应捕获并消灭前，它必须成功感染宿主。
To accomplish this task, viruses have evolved specific interactions with their host species.
要完成这项任务，病毒进化出与宿主物种的特定互动。
Human flu viruses are covered in proteins adapted to bind with matching receptors on human respiratory cells.
人流感病毒覆盖着蛋白质，这种蛋白质可以与人类呼吸细胞上的匹配受体结合。
Once inside a cell, the virus employs additional adaptations to hijack the host cell's reproductive machinery and replicate its own genetic material.
一旦进入细胞，病毒就会采用其他适应措施来劫持宿主细胞的分裂机制，并复制自己的基因物质。
Now the virus only needs to suppress or evade the host's immune system long enough to replicate to sufficient levels and infect more cells.
现在病毒只需压制或躲避宿主的免疫系统，一直繁殖到足够多的水平来感染更多的细胞。
At this point, the flu can be passed on to its next victim via any transmission of infected bodily fluid.
此时，流感可通过任何传染性体液传播，并传递给其下一个受害者。
However, this simple sneeze also brings the virus in contact with pets, plants, or even your lunch.
然而，简单一个喷嚏还会使病毒与宠物、植物甚至你的午餐接触。
Viruses are constantly encountering new species and attempting to infect them. More often than not, this ends in failure.
病毒不断遇到新物种并试图感染它们，这通常会以失败告终。
In most cases, the genetic dissimilarity between the two hosts is too great.
多数情况下，两个宿主间的遗传基因差异太大。
For a virus adapted to infect humans, a lettuce cell would be a foreign and inhospitable landscape.
对于已经适应感染人类的病毒，莴苣细胞则是外来的、且不适宜生存的环境。
But there are a staggering number of viruses circulating in the environment, all with the potential to encounter new hosts.
但环境中传播的病毒数量惊人，极有可能遇到新的宿主。
And because viruses rapidly reproduce by the millions, they can quickly develop random mutations.
而且，因为病毒以数百万级迅速繁殖，他们可以快速发生随机突变。
Most mutations will have no effect, or even prove detrimental; but a small proportion may enable the pathogen to better infect a new species.
大多数突变都没有效果，甚至证明是有害的；但一小部分变异可使病原体更容易感染新物种。
The odds of winning this destructive genetic lottery increase over time, or if the new species is closely related to the virus' usual host.
获得这种破坏性基因的几率会随着时间的推移而增加，或者新物种与病毒的常见宿主相似。
For a virus adapted to another mammal, infecting a human might just take a few lucky mutations.
对于适应一种哺乳动物的病毒，感染人类可能只需一些幸运的突变。
And a virus adapted to chimpanzees, one of our closest genetic relatives, might barely require any changes at all.
一种适应离我们最近的遗传近亲黑猩猩的病毒，可能根本不需要任何突变。
It takes more than time and genetic similarity for a host jump to be successful.
一个宿主成功转移所需要的不仅仅是时间和基因相似性。
Some viruses come equipped to easily infect a new host's cells, but are then unable to evade an immune response.
有些病毒可以轻易感染新宿主细胞，但是无法逃避免疫反应。
Others might have a difficult time transmitting to new hosts.
其他病毒可能较难传染到新宿主。
For example, they might make the host's blood contagious, but not their saliva.
比如，它们可能使宿主的血液具传染性，而非唾液具有传染性。
However, once a host jump reaches the transmission stage, the virus becomes much more dangerous.
但是，一旦宿主转移到传播阶段，病毒变得更加危险。
Now gestating within two hosts, the pathogen has twice the odds of mutating into a more successful virus.
现在病原体可在两个宿主中孕育，其变异为成功病毒的概率提高两倍。
And each new host increases the potential for a full-blown epidemic.
且每个新的宿主都会增加全面流行病的可能性。
Virologists are constantly looking for mutations that might make viruses such as influenza more likely to jump.
病毒学家一直在寻找可能令流感类病毒更易产生宿主转移的突变。
However, predicting the next potential epidemic is a major challenge.
然而，预测下一个潜在的流行病是一项重大挑战。
There's a huge diversity of viruses that we're only just beginning to uncover.
病毒种类繁多，我们才刚刚开始研究。
Researchers are tirelessly studying the biology of these pathogens.
研究人员正在不知疲倦地研究这些病原体生物学。
And by monitoring populations to quickly identify new outbreaks,
通过监测人群来快速识别新的爆发，
they can develop vaccines and containment protocols to stop these deadly diseases.
他们可以开发疫苗和控制措施来阻止这些致命的疾病。
