For all its peculiar horror, cancer comes with a saving grace. If nothing else can stop a tumor’s mad evolution, the cancer ultimately dies with its host. Everything the malignant cells have learned about outwitting the patient’s defenses — and those of the oncologists — is erased. The next case of cancer, in another victim, must start anew.

尽管谈癌色变，但癌症也有“可取之处”。就算无论如何也无法阻止肿瘤的疯狂生长，癌最后也会和宿主同归于尽。恶性癌细胞从击败患者——和肿瘤科医生——的抵抗过程中学到的一切，都会化为乌有。癌症的下一次攻击，必须从下一个受害者身上重新开始。

Imagine if instead, cancer cells had the ability to press on to another body. A cancer like that would have the power to metastasize not just from organ to organ, but from person to person, evolving deadly new skills along the way.

试想，如果癌细胞有能力传染给另一个个体，癌症将不仅有能力从一个器官转移到另一个器官，还可以从一个人转移到另一个人，同时演化出新的致命技能。

While there is no sign of an imminent threat, several recent papers suggest that the eventual emergence of a contagious human cancer is in the realm of medical possibility. This would not be a disease, like cervical cancer, that is set off by the spread of viruses, but rather one in which cancer cells actually travel from one person to another and thrive in their new location.

虽然没有明确迹象显示这是一个迫在眉睫的威胁，但近期许多论文提出，最终出现具有传染性的人类癌症，在医学上是有可能的。它不会是一种疾病，比如由病毒传播引起的子宫颈癌；而是癌细胞实际上从一个人转移到另一个人身上，并在这个新地方肆意生长。

So far this is known to have happened only under the most unusual circumstances. A 19-year-old laboratory worker who pricked herself with a syringe of colon cancer cells developed a tumor in her hand. A surgeon acquired a cancer from his patient after accidentally cutting himself during an operation. There are also cases of malignant cells being transferred from one person to another through an organ transplant or from a woman to her fetus.

迄今为止，只在极为罕见的状况下发现过这种情况。一名19岁的化验室工作人员被带有结肠癌细胞的注射器戳伤后，她的手上长出一个肿瘤。一位外科医生在手术工作中被意外割伤后，从患者身上感染了癌症。癌细胞通过器官移植从一个人转移到另一个身上的案例，或者从母亲转移到胎儿身上的案例也有很多。

On each of these occasions, the malignancy went no further. The only known cancers that continue to move from body to body, evading the immune system, have been found in other animals. In laboratory experiments, for instance, cancer cells have been transferred by mosquitoes from one hamster to another. And so far, three kinds of contagious cancers have been discovered in the wild — in dogs, Tasmanian devils and, most recently, in soft shell clams.

在每一桩这样的案例中，癌细胞都存活不了太久。所有现在已知的能躲过免疫系统，在个体之间转移的癌症，都是在其他动物身上发现的。比如，在实验室里，癌细胞通过蚊子从一只仓鼠转移到另一只身上。目前，在野外环境中发现了三种传染性癌症，分别发生在犬类、袋獾，以及砂海螂身上，后者是最近发现的。

The oldest known example is a cancer that spreads between dogs during sexual intercourse — not as a side effect of a viral or bacterial infection, but rather through direct conveyance of cancer cells. The state of the research is described in a review, “The Cancer Which Survived,” published last year by Andrea Strakova and Elizabeth P. Murchison of the University of Cambridge.

我们所知道的最早的例子是一种癌在犬类之间通过交媾转移的——不是感染病毒或细菌后导致的副作用，而是癌细胞直接从一只犬转移到另一只犬体内。剑桥大学的安德烈娅·斯特拉科娃(Andrea Strakova)和伊丽莎白·P·默奇森(Elizabeth P. Murchison)去年发表的一篇名为《离开宿主依然存活的癌症》的综述文章，对相关研究进行了梳理。

The condition, canine transmissible venereal tumor disease, is believed to have sprung into existence 11,000 years ago — as a single cell in a single dog — and has been circulating ever since. (Why did this happen in dogs and not, say, cats? Perhaps because of what the authors demurely call the dogs’ “long-lasting coital tie” — the half an hour or so that a male and female are locked in intercourse, tearing genital tissues and providing the cancer cells with a leisurely crossing.)

这种名为犬类生殖器传染性肿瘤的疾病，被认为是在1.1万年前突然出现在一只犬身上，而且以一个单一癌细胞的形式一直传播至今。（为什么这种病会出现在犬类身上，而不是在猫等其他动物身上？原因或许在于作者用文明措辞所说的犬类之间“持久的交媾联系”，即在一只母犬和一只公犬持续约半小时的交媾过程中，会撕裂生殖器官，使肿瘤细胞比较容易通过伤口发生转移。）

Normally a cancer evolves in a single body over the course of years or decades, accumulating the mutations that drive it to power. But to have survived for millenniums, researchers have proposed, canine cancer cells may have developed mechanisms — like those in healthy cells — to repair and stabilize their own malignant genomes.

通常一种癌症会在几年或几十年的时间里一直在一个生物体内发展，突变积累到一定程度，就会开始显现威力。但有研究人员提出，这种已经存在上万年之久的犬类癌症细胞，有可能发展出了一些机制——就像健康细胞一样——可以修复和稳定自身的恶性基因。

Early on, cancer cells typically flourish by disabling DNA repair and ramping up the mutational frenzy. Somewhere along the way, the age-old canine cells may have reinvented the device to extend their own longevity. There is also speculation that this cancer may have learned to somehow modify canine sexual behavior in ways that promote the disease’s spread and survival.

早前，癌症细胞往往是通过破坏DNA修复功能而快速生长，引发疯狂的突变。中间的某个时刻，古老的犬类生殖器传染性肿瘤细胞可能改变了行为方式，由此延长自身的寿命。还有一种猜测是，这种癌症可能不知怎么改变了犬类的交媾方式，使之更利于这种疾病的传播和肿瘤细胞的存活。

The second kind of contagious cancer was discovered in the mid-1990s in Tasmanian devils, which spread malignant cells as they try to tear off one another’s faces. Though it may be hard to sympathize, devil facial tumor disease threatens the creatures with extinction.

第二种传染性癌症是在上世纪90年代中期发现于袋獾身上。它们在相互撕咬对方的脸时，会把恶性肿瘤细胞传给其他同类。尽管可能比较难以引起人们的同情，但这种袋獾面部肿瘤疾病的确威胁到了这一物种的生存，使它们濒临灭绝。

With so few examples, transmissible cancer has been easy to dismiss as an aberration. But in December, scientists at the Universities of Tasmania and Cambridge reported in Proceedings of the National Academy of Sciences that Tasmanian devils are passing around another kind of cancer — genetically distinct from the first. It’s weird enough that one such cancer would arise in the species. What are the chances that there would be two?

因为例子非常少，所以可转移癌症很容易被当做一种反常的现象，不予重视。但是去年12月，塔斯马尼亚大学和剑桥大学的科学家们联合发表在《美国国家科学院院刊》(Proceedings of the National Academy of Sciences)上的论文表示，袋獾还在传播另一种癌症，与前一种癌症的基因组成截然不同。这种生物身上会出现一种这样的癌症，就已经够奇怪的了。现在居然说有两种，这种几率有多大？

One theory is that the animals are unusually vulnerable. Driven so close to extinction — by climate change, perhaps, or human predators — the species is lacking in genetic diversity. The cells of another devil injected through a vicious wound may seem so familiar that they are ignored by the recipient’s immune system. If some of the cells carry the mutations for the facial cancer, they might be free to flourish and develop into a new tumor.

有一种理论认为，这种动物非常脆弱。因为气候变化，或许还有人类猎捕的因素，它们已经几乎要灭绝，因此缺乏基因多样性。通过一个严重的伤口进入体内的另一只袋獾的细胞，可能会看似非常熟悉，以至被接收者的免疫系统忽略。如果这些细胞中有一部分携带着面部肿瘤的基因突变，它们可能就会迅速裂变，发展成一种新的肿瘤。

But the scientists also proposed a more disturbing explanation: that the emergence of contagious cancer may not be so rare after all. “The possibility,” they wrote, “warrants further investigation of the risk that such diseases could arise in humans.”

但这些科学家也提出了一种更加让人不安的解释：传染性癌症可能并不那么罕见。“这种可能性，”他们写道，“给了我们进行进一步研究的理由，即研究这类疾病在人类身上出现的风险有多大。”

Cancer has probably existed ever since our first multicellular ancestors appeared on Earth hundreds of millions of years ago. The life spans of even the longest-lived animals may be just too brief for cancers to easily evolve the ability to leap to another body. Otherwise, contagious cancer would be everywhere.

在我们最早的多细胞生物祖先于数亿年前在地球上出现时，癌症可能就已经存在。就算是最长寿的动物，其生命周期对于癌症而言也太过短暂，让它来不及演化出跳转到另一种生物身上的能力。否则，现在就到处都是传染性癌症了。

For now, at least, it remains a curiosity. Consider the case of a 41-year-old man in Medellin, Colombia, who was examined by doctors in 2013 because of fatigue, fever and weight loss. His lymph nodes were clogged with cancer cells that had also spread to his lungs and liver.

至少就目前来说，它还是一种很罕见的现象。比如发生在哥伦比亚麦德林一名41岁男子身上的事。2013年他因出现疲劳、发烧和体重下降等症状而接受医生的检查，结果发现他的淋巴结里长满了癌细胞，还转移到了他的肺部和肝部。

Yet the cells looked far too small and simple to be human. “This case posed a diagnostic conundrum,” the doctors wrote in November in The New England Journal of Medicine.

但是，相对于人类而言，这些癌细胞显得太小，结构也太过简单。“这个病例成了一个难解的谜，”这些医生在去年11月发表于《新英格兰医学杂志》(The New England Journal of Medicine)上的文章中写道。

The solution to the puzzle came when the man was also found to be harboring a tapeworm called Hymenolepis nana. Further analysis concluded that the cancer cells had originated in the parasite and then metastasized through the man’s body.

当这名男子被发现还携带有微膜壳绦虫时，谜底解开了。进一步的分析推断出，这名男子身上的癌细胞最初存在于这种寄生虫内，然后通过他的身体进行了转移。

There is no reason to think that tapeworm cancer is about to become a threat to public health. The patient’s immune system had been compromised by H.I.V., and he died several months later.

没有理由认为，绦虫癌症即将对我们的公共卫生安全构成威胁。这名病人的免疫系统之前就已经因为感染艾滋病毒而遭到了破坏，而且查出这种癌症几个月后，他也去世了。

But nature is infinite in its surprises.

只不过，大自然总会有无限多的意想不到。