It is easy to see why AIDS seemed so mysterious and frightening when US medics first encountered it 35 years ago. The condition robbed young, healthy people of their strong immune system, leaving them weak and vulnerable. And it seemed to come out of nowhere.
35年前，当美国医护人员第一次遇到艾滋病时，艾滋病是如此的神秘和恐怖。它能够摧毁健康年轻人坚固的免疫系统，让机体脆弱易被感染。在那时，没人知道这种疾病是从哪里蹦出来的。
Today we know much more how and why HIV – the virus that leads to AIDS – has become a global pandemic. Unsurprisingly, sex workers unwittingly played a part. But no less important were the roles of trade, the collapse of colonialism, and 20th Century sociopolitical reform.
如今我们对艾滋病，这一世界流行的传染病的传播原因和途径有了更多的了解。除了性工作者不经意地扮演了传播者的角色外，商业贸易、殖民主义的瓦解和20世纪社会政治体制的变革也是同样重要的传播因素。
HIV did not really appear out of nowhere, of course. It probably began as a virus affecting monkeys and apes in west central Africa.
HIV当然不是凭空出现的。HIV很可能是从中非西部的猴子和猿类身上的病毒演变而来的。
From there it jumped species into humans on several occasions, perhaps because people ate infected bushmeat. Some people carry a version of HIV closely related to that seen in sooty mangabey monkeys, for instance. But HIV that came from monkeys has not become a global problem.
HIV可能在若干情形下得以跨物种传播，人类进食了染病的丛林动物或许就是其中一种。例如，曾有患者携带的HIV病毒种类与白颈白眉猴体内的病毒密切相关。
We are more closely related to apes, like gorillas and chimpanzees, than we are to monkeys. But even when HIV has passed into human populations from these apes, it has not necessarily turned into a widespread health issue.
相比猴子，猿类，比如大猩猩和黑猩猩，与人类更加接近。但是即便HIV从猿类传给了人类，也不能说明HIV就能够演化成广泛的卫生问题。
HIV originating from apes typically belongs to a type of virus called HIV-1. One is called HIV-1 group O, and human cases are largely confined to west Africa.
人类从猿类感染的HIV病毒都属于HIV-1型。是一种被称为HIV-1型O组的病毒，病例大部分出现在非洲西部。
In fact, only one form of HIV has spread far and wide after jumping to humans. This version, which probably originated from chimpanzees, is called HIV-1 group M (for "major"). More than 90% of HIV infections belong in group M. Which raises an obvious question: what's so special about HIV-1 group M?
而事实上，真正在人与人之间广泛流行只有一种HIV。这种HIV病毒很可能是从黑猩猩身上传播而来的，它被称为 HIV-1型 M组。90%多的HIV感染者都属于这种病毒的携带者。问题来了：是什么让这种病毒如此特殊呢？
A study published in 2014 suggests a surprising answer: there might be nothing particularly special about group M.
而2014年的一篇研究论文得出了惊人的结论：M组病毒可能并无特殊之处。
It is not especially infectious, as you might expect. Instead, it seems that this form of HIV simply took advantage of events. "Ecological rather than evolutionary factors drove its rapid spread," says Nuno Faria at the University of Oxford in the UK.
M组病毒并不像想象的那样有特别高的传染性。它似乎只是很好地利用了各种可以传播的机会。“M组病毒得以迅速传播依靠的是生态学因素而非进化学因素”，英国牛津大学的努诺 法利亚（Nuno Faria）说。
Faria and his colleagues built a family tree of HIV, by looking at a diverse array of HIV genomes collected from about 800 infected people from central Africa.
通过对非洲中部800名感染者的各种基因组取样研究，法利亚和同事们画出了HIV的家族图谱。
Genomes pick up new mutations at a fairly steady rate, so by comparing two genome sequences and counting the differences they could work out when the two last shared a common ancestor. This technique is widely used, for example to establish that our common ancestor with chimpanzees lived at least 7 million years ago.
基因组能够相当稳定地记录基因的突变，所以通过比对两组基因序列的异同，研究人员便可以得出两组基因序列最近一次的共同祖先。这一技术已经被广泛应用。例如，人类与黑猩猩的共同祖先生活在距今700万年前的结论就是应用这一技术得出的。
"RNA viruses such as HIV evolve approximately 1 million times faster than human DNA," says Faria. This means the HIV "molecular clock" ticks very fast indeed.
“像HIV这样的核糖核酸病毒的进化速度大约是人类DNA进化速度的100万倍”，法利亚说。这就意味着HIV的“分子钟”走得飞快。
It ticks so fast, Faria and his colleagues found that the HIV genomes all shared a common ancestor that existed no more than 100 years ago. The HIV-1 group M pandemic probably first began in the 1920s.
法利亚和同事们发现HIV的进化速度非常快，它们基因组的最近一次共同祖先出现在距今区区100年前。而HIV-1型M组艾滋病也很可能于1920年代才开始出现。
Then the team went further. Because they knew where each of the HIV samples had been collected, they could place the origin of the pandemic in a specific city: Kinshasa, now the capital of the Democratic Republic of Congo.
得出该结论后，研究组继续深入研究。因为知道每个HIV样本的采集地点，所以他们可以推算出病源的具体城市：M组的病源城市很可能就是刚果民主共和国首都金沙萨。
At this point, the researchers changed tack. They turned to historical records to work out why HIV infections in an African city in the 1920s could ultimately spark a pandemic.
至此，研究员们改变了研究路径。他们开始研究为什么1920年代在这座非洲城市出现的HIV最后演变成了流行性疾病。
A likely sequence of events quickly became obvious.
一系列推论很快变得明朗起来。
In the 1920s, DR Congo was a Belgian colony and Kinshasa – then known as Leopoldville – had just been made the capital. The city became a very attractive destination for young working men seeking their fortunes, and for sex workers only too willing to help them spend their earnings. The virus spread quickly through the population.
上世纪20年代，刚果民主共和国还是比利时的殖民地，金沙萨—那时还被叫做利奥波德维尔—刚刚被定为首府。当时这座城市吸引了无数青年男性来挣钱，而性工作者让他们找到了花钱的地方。病毒就这样迅速蔓延开来。
It did not remain confined to the city. The researchers discovered that the capital of the Belgian Congo was, in the 1920s, one of the best connected cities in Africa. Taking full advantage of an extensive rail network used by hundreds of thousands of people each year, the virus spread to cities 900 miles (1500km) away in just 20 years.
病毒的蔓延不只局限于这座城市。研究人员发现，20年代，比利时统治下的刚果首府是非洲交通最频繁的城市之一。每年成千上万的人们坐着火车经过这里，病毒很好的利用了这点，在仅仅20年的时间里蔓延到了距离金沙萨900英里（1500公里）远的其他城市中。
Everything was in place for an explosion in infection rates in the 1960s.
到了上世纪60年代，艾滋病的大爆发已是万事俱备。
The beginning of that decade brought another change.
60年代初世界又发生了新的变化。
Belgian Congo gained its independence, and became an attractive source of employment to French speakers elsewhere in the world, including Haiti. When these young Haitians returned home a few years later they took a particular form of HIV-1 group M, called "subtype B", to the western side of the Atlantic.
刚果从比利时人手中获得了独立，变成了世界上说法语的人，比如海地人，寻找就业机会的圣地。当这些海地人在刚果工作几年后，回到家乡，同时也将HIV-1型M组病毒的一种亚型体“subtype B”带到了大西洋西岸。
It arrived in the US in the 1970s, just as sexual liberation and homophobic attitudes were leading to concentrations of gay men in cosmopolitan cities like New York and San Francisco. Once more, HIV took advantage of the sociopolitical situation to spread quickly through the US and Europe."There is no reason to believe that other subtypes would not have spread as quickly as subtype B, given similar ecological circumstances," says Faria.
1970年代，随着性解放运动和反同性恋思潮将大都市中的男同性恋群体推向风口浪尖，这种亚型病毒也传入了美国。HIV这次又很好地抓住了一次社会政治变动所带来的机会将自己传遍美国和欧洲。“我们有理由相信其他亚型HIV也会像subtpye B 一样在相似的社会生态系统下得到快速的传播”, 法利亚说。
The story of the spread of HIV is not over yet.
HIV的传播还在继续。
For instance, in 2015 there was an outbreak in the US state of Indiana, associated with drug injecting.
例如，2015年美国印第安纳州的艾滋病爆发就与毒品注射有关。
The US Centers for Disease Control and Prevention has been analyzing the HIV genome sequences and data about location and time of infection, says Yonatan Grad at the Harvard School of Public Health in Boston, Massachusetts. "These data help to understand the extent of the outbreak, and will further help to understand when public health interventions have worked."
美国疾控中心一直在分析患者的HIV基因组序列和感染的时间地点数据，哈弗大学波士顿公共卫生学院约纳坦 格兰特（Yonatan Grad）说。“这些数据有助于我们了解病情爆发的范围，还有助于我们了解何时采取公共卫生干预工作最有效。”
This approach can work for other pathogens. In 2014, Grad and his colleague Marc Lipsitch published an investigation into the spread of drug-resistant gonorrhoea across the US.
这种研究方法同样适用于针对其他病原体的研究。2014年，格兰特和同事马克（Marc Lipsitch）发表了全美耐药性淋病传播的调查研究。
"Because we had representative sequences from individuals in different cities at different times and with different sexual orientations, we could show the spread was from the west of the country to the east," says Lipsitch.
“我们掌握了不同城市、不同时间和不同性取向的患者基因序列，得出这次疾病是从美国西部向东部传播的”，马克说。
What's more, they could confirm that the drug-resistant form of gonorrhoea appeared to have circulated predominantly in men who have sex with men. That could prompt increased screening in these at-risk populations, in an effort to reduce further spread.
而且他们还通过分析这些数据得出耐药性淋病在男同性恋群体中的传播尤为显著。这一发现有助于对高危群体施以更多的监看，降低疾病进一步传播的风险。
In other words, there is real power to studying pathogens like HIV and gonorrhoea through the prism of human society.
换言之，通过人类社会行为研究HIV和淋病之类的病原体是行之有效的方法。
By Colin Barras
柯林 巴拉斯著
