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試試At a Maryland country fair in 2017, the prize pigs were not looking their best. Farmers reported feverish hogs with inflamed eyes and running snouts.
在馬里蘭州2017年的郡博覽會上，獲獎的豬看起來狀態並不好。農民報告說，這些發燒的豬 眼睛紅腫、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. 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.
但是，雖然展會官員擔心這些豬，但馬里蘭州衛生部門則對一組生病的參會者很重視。有些人摸過這些豬，而有些人僅從它們的圈旁路過；但很快，這些參與者中40人將被診斷患豬流感。
More often than not, sick animals don't infect humans. But when they do, these cross-species infections, or viral host jumps, have the potential to produce deadly epidemics.
生病動物通常不會感染人類。但是當它們傳染時，這些跨物種感染，也叫病毒宿主轉移，可能產生致命的流行病。
So how can pathogens from one species infect another, and what makes host jumps so dangerous? Viruses are a type of organic parasite infecting nearly all forms of life.
那麼一個物種的病原體如何感染另一個物種的？什麼使宿主轉移如此危險呢？病毒是一種感染幾乎所有生命形式的有機寄生物。
To survive and reproduce, they must move through three stages: contact with a susceptible host, infection and replication, and transmission to other individuals. As an example, let's look at human influenza.
為了生存和繁殖，它們必須經歷三個階段：與易感宿主接觸、感染和複製、以及傳遞給其他人。舉個例子，我們來看看人類流感。
First, the flu virus encounters a new host and makes its way into their respiratory tract. This isn't so difficult, but to survive in this new body, the virus must mount a successful infection before it's caught and broken down by an immune response.
首先，流感病毒遇到了新的宿主，並進入他們的呼吸道。進入並不困難，但要在這個新的身體中生存，在病毒被免疫反應捕獲並消滅前，它必須成功感染宿主。
To accomplish this task, viruses have evolved specific interactions with their host species. Human flu viruses are covered in proteins adapted to bind with matching receptors on human respiratory cells.
要完成這項任務，病毒進化出與宿主物種的特定互動。人流感病毒覆蓋著蛋白質，這種蛋白質可以與人類呼吸細胞上的匹配受體結合。
Once inside a cell, the virus employs additional adaptations to hijack the host cell's reproductive machinery and replicate its own genetic material. 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.
一旦進入細胞，病毒就會採用其他適應措施來劫持宿主細胞的分裂機制，並複製自己的基因物質。現在病毒只需壓制或躲避宿主的免疫系統，一直繁殖到足夠多的水平來感染更多的細胞。
At this point, the flu can be passed on to its next victim via any transmission of infected bodily fluid. However, this simple sneeze also brings the virus in contact with pets, plants, or even your lunch.
此時，流感可通過任何傳染性體液傳播，並傳遞給其下一個受害者。然而，簡單一個噴嚏還會使病毒與寵物、 植物甚至你的午餐接觸。
Viruses are constantly encountering new species and attempting to infect them. More often than not, this ends in failure.
病毒不斷遇到新物種並試圖感染它們。這通常會以失敗告終。
In most cases, the genetic dissimilarity between the two hosts is too great. For a virus adapted to infect humans, a lettuce cell would be a foreign and inhospitable landscape.
多數情況下，兩個宿主間的 遺傳基因差異太大。對於已經適應感染人類的病毒，萵苣細胞則是外來的、 且不適宜生存的環境。
But there are a staggering number of viruses circulating in the environment, all with the potential to encounter new hosts.And because viruses rapidly reproduce by the millions, they can quickly develop random mutations.
但環境中傳播的病毒數量驚人，極有可能遇到新的宿主。而且，因為病毒以數百萬級迅速繁殖，他們可以快速發生隨機突變。
Most mutations will have no effect, or even prove detrimental; but a small proportion may enable the pathogen to better infect a new species. The odds of winning this destructive genetic lottery increase over time, or if the new species is closely related to the virus' usual host.
大多數突變都沒有效果，甚至證明是有害的；但一小部分變異可使病原體更容易感染新物種。獲得這種破壞性基因的幾率會隨著時間的推移而增加，或者新物種與病毒的常見宿主相似。
For a virus adapted to another mammal, infecting a human might just take a few lucky mutations. And a virus adapted to chimpanzees, one of our closest genetic relatives, might barely require any changes at all.
對於適應一種哺乳動物的病毒，感染人類可能只需一些幸運的突變。一種適應離我們最近的遺傳近親黑猩猩的病毒，幾乎不需要任何突變。
It takes more than time and genetic similarity for a host jump to be successful. Some viruses come equipped to easily infect a new host's cells, but are then unable to evade an immune response.
一個宿主成功轉移所需要的不僅僅是時間和基因相似性。有些病毒可以輕易感染新宿主細胞，但是無法逃避免疫反應。
Others might have a difficult time transmitting to new hosts. For example, they might make the host's blood contagious, but not their saliva.
其他病毒可能較難傳染到新宿主。比如，它們可能使宿主的血液具傳染性，而非唾液具有傳染性。
However, once a host jump reaches the transmission stage, the virus becomes much more dangerous. Now gestating within two hosts, the pathogen has twice the odds of mutating into a more successful virus.
但是，一旦宿主轉移到傳播階段，病毒變得更加危險。現在病原體可在兩個宿主中孕育，其變異為成功病毒的概率提高兩倍。
And each new host increases the potential for a full-blown epidemic. Virologists are constantly looking for mutations that might make viruses such as influenza more likely to jump.
且每個新的宿主都會增加全面流行病的可能性。病毒學家一直在尋找可能令流感類病毒更易產生宿主轉移的突變。
However, predicting the next potential epidemic is a major challenge. There's a huge diversity of viruses that we're only just beginning to uncover.
然而，預測下一個潛在的流行病是一項重大挑戰。病毒種類繁多，我們才剛剛開始研究。
Researchers are tirelessly studying the biology of these pathogens. And by monitoring populations to quickly identify new outbreaks, they can develop vaccines and containment protocols to stop these deadly diseases.
研究人員正在不知疲倦地研究這些病原體生物學。並通過監測人群來快速識別新的爆發，他們可以開發疫苗和控制措施來阻止這些致命的疾病。
