流行病学 1855

论霍乱的传播方式

约翰·斯诺

他把一场霍乱疫情追溯到一口水泵——并证明疾病能藏在水里传播，而不只在空气中。

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In depth · the introduction

一位医生，凭一张街道地图和一份死者名单，发现一场霍乱疫情正从街区里的一口水泵涌出。

核心想法

1854 年，多数医生把霍乱归咎于「瘴气」——从污秽中升腾起来的坏空气。约翰·斯诺不以为然。他认为这种病是被吞下去的：一种看不见的毒物，随病人的排泄物传出，而最主要的，是藏在脏的饮水里。

为了检验，他做了一件新鲜事。他没有就空气争来争去，而是清点死者，把每一个都标到他们苏豪区街道的地图上，并向每户人家问一个朴素的问题：你家的水从哪里来？答案一次又一次，指向布罗德街上的同一口公用水泵。

它是如何诞生的

斯诺当时已是一位声名卓著的医生——他曾为维多利亚女王分娩施用氯仿，助推了现代麻醉术的诞生。但霍乱才是他的执念。1854 年 8 月底，霍乱在苏豪区暴发，数日内夺去数百人性命，他便走街串巷，一面收集住址，一面记下取水来源。他的地图显示，死亡密密匝匝地围在布罗德街水泵周围，却放过了那些在别处取水的人：一座有自家水井的济贫院、一家工人只喝啤酒的酿酒厂，几乎毫发无损。

9 月 7 日傍晚，他说服了教区委员会，把那口水泵的手柄拆了下来。他还在南伦敦做了一场更大的检验：那里有两家公司，把水卖给沿同一条条街道交错杂居的人家——一家取自泰晤士河被污水浸染的一段，另一家取自上游较洁净的水。斯诺挨家敲门，查明每一例霍乱死者由哪家公司供水。喝洁净水的房屋安然无恙；喝脏水的房屋，死亡率高出数倍。

它为何重要

斯诺是对的，而在「水」这件事上对了，所挽救的生命比他那个时代的任何药物都多。一旦城市接受了霍乱、伤寒与痢疾会随被污水沾染的水传播，它们便修起下水道、过滤供水、看护水井——正是这些，终结了富裕世界里的这类疫情。他由此奠定了流行病学——追踪疾病如何在整个人群中蔓延的科学——并表明：公共卫生，是可以被「营造」出来的。

一个可以想象的画面

把一座城镇的水想成它共享的邮件。如果一封带传染性的信被投进了整条街都从中取阅的同一个邮袋，那么凡打开这个袋子的人都会被波及——无论各家相距多远。斯诺的高招，就是不再争论天气，转而去问：每个病人，是从哪个袋子里取的？追那个袋子，而非那阵风，死亡便会在一处被污染的源头周围，骤然收拢成一圈。

它的位置

斯诺的工作，比能为他作证的细菌学说早了一整代人。路易·巴斯德与罗伯特·科赫不久便将证明：特定的微生物引起特定的疾病，而科赫终于在 1883 年捕获了霍乱菌。斯诺仅凭模式就推到了同样的结论——正如孟德尔从清点豌豆，推断出他看不见的基因。他的地图开启的那条线索，一直通到今天的疾病侦探：面对一场新的疫情，他们依旧用清点、绘图、顺水追查来应对。

The original document

Original source text

John Snow · On the Mode of Communication of Cholera, 2nd ed. · London: John Churchill, 1855

Snow's thesis (in summary): against the reigning belief that cholera rose on poisoned air, he held that it is a disease of the alimentary canal — a specific 'morbid material' voided by the sick, swallowed by the well in water fouled with sewage, and multiplying within the new victim's gut. A little, therefore, could sicken a city.

The Broad Street pump · Soho, 1854

The most terrible outbreak of cholera which ever occurred in this kingdom, is probably that which took place in Broad Street, Golden Square, and the adjoining streets, a few weeks ago.

Marking each death on a map of the parish, Snow found them clustered tightly about one public well — the pump in Broad Street — and thinning with distance from it. He chased every exception until it confirmed the rule.

There were only ten deaths in houses situated decidedly nearer to another street-pump. In five of these cases the families of the deceased persons informed me that they always sent to the pump in Broad Street, as they preferred the water to that of the pumps which were nearer.

I had an interview with the Board of Guardians of St. James's parish, on the evening of Thursday, 7th September, and represented the above circumstances to them. In consequence of what I said, the handle of the pump was removed on the following day.

[ … ]

The Grand Experiment · South London

South of the Thames, two companies sold piped water to houses intermingled along the same streets: the Southwark & Vauxhall Company, drawing from a sewage-laden reach of the river, and the Lambeth Company, whose intake had moved upstream to cleaner water. Families alike in every other respect were thereby sorted by their water alone — a natural experiment Snow seized.

The experiment, too, was on the grandest scale. No fewer than three hundred thousand people of both sexes, of every age and occupation, and of every rank and station, from gentlefolks down to the very poor, were divided into two groups without their choice, and, in most cases, without their knowledge; one group being supplied with water containing the sewage of London, and, amongst it, whatever might have come from the cholera patients, the other group having water quite free from such impurity.

Snow's tally bore the prediction out: among the 40,046 houses supplied by Southwark & Vauxhall there were 1,263 cholera deaths — 315 per 10,000 houses — against 98 deaths in the 26,107 houses supplied by Lambeth, just 37 per 10,000. The mortality on the sewage-fed water was, in his words, 'between eight and nine times as great.'

London · John Churchill · 1855