It had started somewhere in China, or Mongolia perhaps, emerging like an ancient locust from the soil. Shadowing the Silk Road, it had crossed the vast Eurasian steppe, traversed the Anatolian peninsula, and arrived in Crimea on the Black Sea. Everywhere, death had followed.

    At the Black Sea port of Caffa, where merchants from the Italian city of Genoa had set up a trading outpost, the pestilence landed with a fury. The Genoese fled to their ships, racing back through the Black Sea, to the Sea of Marmara, then to the Mediterranean. Unknowingly, they brought the sickness and fury with them. From ship to port—Genoa, Venice, the Sicilian town of Messina, the French harbor of Marseille—the microscopic Yersinia pestis followed, hiding in the stomachs of Oriental rat fleas, circulating in the blood of their prey, the black rat, and within each new human being infected.

    The illness came in various forms—bubonic, pneumonic, septicemic. All three were swift, painful and gruesome, with many of the victims developing oozing tumors in their groins, armpits, or necks.

    它最早起源于中国或蒙古的某个地方，仿佛像一只突然从黄沙中钻出来的蚱蜢。它沿着丝绸之路，跨过广阔的欧洲大草原，横穿安那托利亚半岛，最后来到位于黑海之滨的克里米亚。所到之处，死亡如影随形。

    黑海之滨的卡法港一度是热那亚商人的贸易前哨地。这场瘟疫爆发后，热那亚商人们抱头鼠窜地跳上商船，通过黑海狂奔回马尔马拉海，然后进入地中海。但不知不觉地，他们也带回了瘟疫。死神随着他们下船登岸，从热那亚、威尼斯、墨西拿的西西里镇一直扫荡到法国的马赛。隐藏在东方鼠蚤肠胃内的鼠疫杆菌随同其猎物（黑家鼠）的血液四处传播，同时也进入每一个被感染的人体内。

    这种病菌有多种症状体征，比如淋巴腺肿大、肺炎、败血症等等。这三种症状都有发病快、痛苦、致死率高的特点，很多病人的腹股沟、腋下、颈部淋巴都会冒出肿瘤。每天都有几千人死亡。

    It took only weeks for the disease to arrive in Pisa, a center of trade on the Tyrrhenian Sea. It took mere months to carve through Eastern Spain, France and Germany, slicing through the population as if clear-cutting a forest. Ships from Gascony in Southwestern France hauled the disease to Melcombe Regis, a small seaport on England’s southern flank. From there, it was carried to crowded Bristol, a month later to London, and before long to all the British Isles, Norway, Denmark, as far east as Russia.

    “The Black Death,” it was called; yet even more fitting was the name, the “Great Mortality”: Over the course of some five years in the mid-14th century, the mysterious plague claimed the lives of tens of millions of people—a reign of terror that, by most assessments, killed between a quarter and half the population of Europe.

    This wasn’t the first time a massive, globe-spanning pandemic had taken hold: Eight centuries earlier, the Justinian plague—ostensibly caused by the same microbe—had killed an estimated 100 million people. Nor would it be the last devastating outbreak of Y. pestis. A third major pandemic, the so-called Modern Plague, began in China in the 1860s and claimed at least 10 million lives before it was through.

    What triggered any of them? University of Oslo biologist Nils Christian Stenseth and colleagues have shown that reservoirs of the plague bacteria remain common in much of the world (see map below). And sporadic cases of bubonic plague have popped up as recently as this summer in the U.S., and in China, as they did in Madagascar in December. Could another Black Death take hold and, if so, how far would it spread?

    仅仅几周时间，这种疾病就蔓延到了第勒尼安海的贸易重镇比萨。不到几个月，它便横扫西班牙东部和法德两国。所到之处，十室九空。来自法国加斯科尼的海船又把它载到了英国南端一个名叫梅尔库姆里吉斯的小港口，它从这里登陆，杀向人口众多的英国西部港市布里斯托，一个月后传播到了伦敦。不久后，整个不列颠群岛，加上挪威、丹麦，甚至东到俄罗斯，都成了它的肆虐之所。

    它就是欧洲人谈之色变的“黑死病(The Black Death)”。或许更适合的名称应该是“大死亡(Great Mortality)”。在14世纪中叶大约5年的时间里，这种神秘的瘟疫导致数千万人丧命，也就是说，死神的镰刀一举划掉了当时整个欧洲四分之一到一半的人口。

    黑死病其实并不是第一起在全球范围内大规模流行的瘟疫。在此之前约8个世纪，查士丁尼大瘟疫大约导致1亿人丧命，这种瘟疫据说也是鼠疫。同时黑死病也不是鼠疫在人类历史上最后一次肆虐。第三次鼠疫大流行又叫“现代瘟疫”，起源于19世纪60年代的中国，至少导致了1000万人死亡。

    那么，这些瘟疫究竟是由什么触发的？奥斯陆大学（University of Oslo）生物学家尼尔斯•克里斯蒂安•斯坦瑟斯及其同事指出，鼠疫病菌的宿主生物在全球各地都不鲜见（见下图），而且偶发的鼠疫病例直到今年夏天还曾在中国和美国出现，去年12月，马达加斯加也出现了人感染鼠疫的病例。那么这是否意味着黑死病或许又将卷土重来？如果答案是肯定的，这次它会流传多广？

    The Global Distribution of Plague (A) Map showing countries with known presence of plague in wild reservoir species (red) (after [3]). For US only the mainland below 50° N is shown. (B) Annual number of human plague cases over different continents, reported to WHO in the period 1954–2005. (C) Cumulative number of countries that reported plague to WHO since 1954.

    全球鼠疫分布图（A图）显示的是存在鼠疫病菌宿主物种的国家（红色部分）。美国只标出了北纬50度以下的部分。B图显示世界卫生组织所掌握的1954到2005年间各大洲每年人感染鼠疫的病例。C图显示自1954年以来出现过鼠疫病例的国家的累计数。

    Experts have warned that climate change might increase the risk of outbreaks in areas where the Y. pestis bacillus is already endemic and that new danger zones may emerge. But it’s all guesswork now. “Remarkably little is known about the dynamics of plague in its natural reservoirs and hence about changing risks for humans,” wroteStenseth and his colleagues in the journal PLOS Medicine. “Plague should be taken much more seriously by the international community than appears to be the case.”

    Yes, centuries after the Black Death, the actual illness at the center of the most famous and studied pandemic in history remains shrouded in mystery. And, importantly, so too does the process of contagion itself, a cascading ripple effect that turns out to be everywhere. Our yawns, our moods, our facial expressions, our memories, our office dynamics, our global banking system are all subject to the forces of contagion, it would seem.

    The question is, whether studying one manifestation of the process can shed light on another. Can researching the way pathogens spread, for instance, help us understand financial contagions? (It may be telling that financial crises in 1997 and 1998 were known, respectively, as the “Asian Flu” and the “Russian Virus.”) Or can making sense of “group emotional contagion”—how moods are transferred among individuals in, say, an office or on a sports team—give us insights into preventing “outbreaks” of suicide?

    With such aspirations in mind, Fortune set out to explore how various things spread. We asked five writers to delve into seven manifestations of contagion—some familiar, as in the global spread of a virus; some, a bit unusual—and we’ve gathered their essays here (see the links below).

    In a pair of posts, Erika Fry begins by investigating how a deadly new coronavirus, the MERS-CoV, emerged in Saudi Arabia, traveled from bats to camels to humans, and ended up in a small community hospital in Munster, Indiana. As the world nervously keeps watch on the epidemic of Ebola, now raging through parts of West Africa, Fry shows why the Saudi coronavirus (a cousin of the infamous SARS virus) has, perhaps, a greater potential to spread.

    Jen Wieczner and Stephen Gandel each probe different ends of stock market contagion: Wieczner takes us deep inside the corporate takeover rumor mill, showing how the thinnest reed of conjecture can quickly become stock-jolting gossip; and Gandel, a veteran financial reporter, dives into one of the enduring mysteries of the market—what triggers a sudden broad stock selloff when there’s seemingly no precipitating news? In other words, how exactly does that collective “panic button” get pushed?

    If you’ve ever wondered how the term “runaway” ever got hitched to the word “bestseller,” then read Anne VanderMey’s essay on Thomas Piketty’s Capital in the Twenty-First Century, the bonafide nonfiction contagion of 2014. Jessi Hempel, meanwhile, takes a turn exploring another, surprisingly durable, social fad: the “selfie.” As Hempel discovers, the meteoric rise of the insta-self-portrait is due as much to transformative changes in technology as it is to human compulsion…and, well, narcissism. And finally, Erika Fry returns to investigate the investigators: showing how studying viral trends on Twitter and other social media got to be such a viral academic specialty. Welcome to Twitterology 101: the hot ticket on campus.

    Adding fuel to each of these contagions is our ever-growing web of connections to the global village, with the virtual tethers now so much a part of our daily lives that they no longer surprise. Every Facebook user, in theory, is just a single friend request away from some 1.3 billion others. Each of nearly 300 million LinkedIn members is a mere click away from an old colleague, a new professional contact, a stranger’s query.

    Even when it comes to that old-world concept of human contact, we are more interconnected than ever. Consider the more than 3 billion passengers in the world who will likely travel on an airline this year (based on last year’s figures). With each voyage in the clouds will come an opportunity for a new link or exchange with a stranger—a conversation, a business card, a spilled coffee, a stowaway germ.

    Which, again, puts the Black Death of the Middle Ages in stark perspective. That such a far-flung pandemic—blanketing an area of roughly 7 million square kilometers—could happen centuries before the invention of the airplane, the car, the truck, or the train is astounding. (Indeed, the uncanny speed and scope of its spread has led some experts to wonder whether perhaps they’ve fingered the wrong pathogen in that outbreak, and whether the real driver might have been a more readily airborne virus.)

    We are, of course, just beginning to understand (and imagine) how contagion may manifest in an age when each of us is potentially just an email, or YouTube video, or middle seat away from the rest of the world. Here, some eye-opening perspectives on how it has revealed itself so far.

    专家警告称，对于一些鼠疫已经在小范围流行的地区来说，气候变化很可能会加剧鼠疫爆发的风险，新的危险区也有可能随之出现。但目前这还只是猜测。斯坦瑟斯及其同事在《PLOS医学》（PLOS Medicine）杂志上撰文指出：“人们对鼠疫病菌在天然宿主体内的发展及其变异对人体的风险所知甚少。国际社会应该比现在更加重视鼠疫问题。”

    是的，在黑死病大爆发几百年后，这一史上最著名、被反复研究的瘟疫依然笼罩在神秘之中。与此同时，我们对“传染”这个过程本身的了解也远远不够。比如说打呵欠也是会传染的，再比如说我们的情绪、面部表情、记忆、办公室的活力，乃至全球银行系统，都容易受到传染力量的影响。

    问题是，对一种传染形式的研究是否有助于我们了解其他传染机制？比如通过研究病菌的传染，能否有助于我们研究金融危机的传染？（1997年和1998年的经济危机又分别叫做“亚洲流感”和“俄罗斯病毒”。）抑或，通过研究“集体情绪传染”（即情绪如何在个体间传染），能否有助于我们预防自杀的“井喷”？

    抱着这样的愿望，《财富》（Fortune）决定探索各种事物的传播机制。我们邀请五位作者深入探究了传染的几种表现形式——有些是比较熟悉的，比如病毒的全球传播；有些则有点不太寻常，但我们收集了相关论文（见下方链接）。

    在两篇文章中，埃瑞卡•弗莱首先调查一种在沙特阿拉伯爆发，名叫MERS-CoV的冠状病毒，是如何从蝙蝠传染到骆驼、再从骆驼传染到人类，最后又出现在印第安那州孟斯特一家小型社区医院的。这种病毒是臭名昭著的SARS病毒的近亲。就在全球正在紧张关注正在西非部分地区肆虐的埃博拉病毒之际，弗莱向我们展示了为什么MERS-CoV可能具有更大的蔓延风险。

    珍•韦茨纳和史蒂芬•甘代尔分别从不同方面研究了股市传染病。韦茨纳首先深入研究了并购圈的谣言传播路线图，展示了一次简单的猜测如何演变成引发股市震荡的流言。具有多年金融报道经验的甘代尔则深入探究了股市上一个多年未解之谜——为什么在没有任何重大负面消息的情况下，会突然出现大范围的抛售股票？换句话说，大家的“恐慌按钮”究竟是如何被人按下的？

    如果你想知道“逃亡”一词是如何跟“畅销书”联系起来的，不妨阅读一下安妮•范德梅评论2014年畅销书——托马斯•皮凯蒂的《21世纪资本论》（Capital in the Twenty-First Century）的论文。杰西•亨佩尔则研究了另一个热门的社会现象——“自拍”。据亨佩尔研究发现，“自拍”的迅速泛滥，既受到科技革命性变革的推动，同时也出于人性的冲动和自恋。最后，埃瑞卡•弗莱又把研究对象转移到了研究者自己身上，阐释了为什么研究Twitter等社交媒体也变成了一种流行的学术行为。欢迎了解大学校园内最热门的学科之一：《Twitter学入门基础》。

    网络的力量既让我们与地球村联系得更加紧密，也使这些“传染病”传播得更加迅速。这张看不见的网已经成为我们日常生活中密不可分的一部分。从理论上讲，每名Facebook用户只需要发送一个交友申请，就可以跟另外13亿人成为朋友。LinkedIn上的近3亿用户，只需要轻轻一点，就可以联系上一个老同事，找到一个新的职业机会，认识一个陌生人。

    即便是用传统意义界定的“人际交往”来看，人们之间的交往也比以往更加紧密了。今年，全球有30多亿人次要通过飞机出行（根据去年的数据）。每次“云端之旅”都有可能让你认识一个陌生人，或者与其他人交换一些东西——比如一次对话、一张名片、一杯不小心溢出的咖啡，或者一种隐藏的病菌。

    中世纪的黑死病再次进入我们的视野。这样一种蔓延极广，横扫了将近700万平方公里区域的流行病，居然发生在飞机、汽车、火车出现的好几百年之前，这实在令人震惊。（的确如此，由于黑死病传播速度之快、范围之广，有些专家甚至怀疑我们或许认错了对象，或许当年爆发的是另一种更致命的、通过空气传播的病毒。）

    在当今世界，我们离整个世界的距离也许只有一封电子邮件、一个YouTube视频、一张飞机座椅那么远。在了解（以及想象）传染病将如何在这样一个时代传播方面，我们只是刚刚迈出了第一步。不过在本系列中，一些令人耳目一新的观点已经开始掀起传染的神秘面纱。（财富中文网）

“传染”系列文章：

【传染之一】比SARS更致命：蝙蝠病毒MERS是如何成为人类杀手的

【传染之二】“自拍”何以变成社会流行病

【传染之三】市场抛盘是怎样发生的？

【传染之四】并购传闻如何不胫而走

【传染之五】从贾斯汀•比伯到数据学家，Twitter何以成为一门显学

    译者：朴成奎