流行病學 1855

論霍亂的傳播方式

約翰·斯諾

他把一場霍亂疫情追溯到一口水泵——並證明疾病能藏在水裡傳播，而不只在空氣中。

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In depth · the introduction

一位醫生，憑一張街道地圖和一份死者名單，發現一場霍亂疫情正從街區裡的一口水泵湧出。

核心想法

1854 年，多數醫生把霍亂歸咎於「瘴氣」——從汙穢中升騰起來的壞空氣。約翰·斯諾不以為然。他認為這種病是被吞下去的：一種看不見的毒物，隨病人的排泄物傳出，而最主要的，是藏在髒的飲水裡。

為了檢驗，他做了一件新鮮事。他沒有就空氣爭來爭去，而是清點死者，把每一個都標到他們蘇活區街道的地圖上，並向每戶人家問一個樸素的問題：你家的水從哪裡來？答案一次又一次，指向布羅德街上的同一口公用水泵。

它是如何誕生的

斯諾當時已是一位聲名卓著的醫生——他曾為維多利亞女王分娩施用氯仿，助推了現代麻醉術的誕生。但霍亂才是他的執念。1854 年 8 月底，霍亂在蘇活區暴發，數日內奪去數百人性命，他便走街串巷，一面收集住址，一面記下取水來源。他的地圖顯示，死亡密密匝匝地圍在布羅德街水泵周圍，卻放過了那些在別處取水的人：一座有自家水井的濟貧院、一家工人只喝啤酒的釀酒廠，幾乎毫髮無損。

9 月 7 日傍晚，他說服了教區委員會，把那口水泵的手柄拆了下來。他還在南倫敦做了一場更大的檢驗：那裡有兩家公司，把水賣給沿同一條條街道交錯雜居的人家——一家取自泰晤士河被汙水浸染的一段，另一家取自上游較潔淨的水。斯諾挨家敲門，查明每一例霍亂死者由哪家公司供水。喝潔淨水的房屋安然無恙；喝髒水的房屋，死亡率高出數倍。

它為何重要

斯諾是對的，而在「水」這件事上對了，所挽救的生命比他那個時代的任何藥物都多。一旦城市接受了霍亂、傷寒與痢疾會隨被汙水沾染的水傳播，它們便修起下水道、過濾供水、看護水井——正是這些，終結了富裕世界裡的這類疫情。他由此奠定了流行病學——追蹤疾病如何在整個人群中蔓延的科學——並表明：公共衛生，是可以被「營造」出來的。

一個可以想像的畫面

把一座城鎮的水想成它共享的郵件。如果一封帶傳染性的信被投進了整條街都從中取閱的同一個郵袋，那麼凡打開這個袋子的人都會被波及——無論各家相距多遠。斯諾的高招，就是不再爭論天氣，轉而去問：每個病人，是從哪個袋子裡取的？追那個袋子，而非那陣風，死亡便會在一處被汙染的源頭周圍，驟然收攏成一圈。

它的位置

斯諾的工作，比能為他作證的細菌學說早了一整代人。路易·巴斯德與羅伯特·科赫不久便將證明：特定的微生物引起特定的疾病，而科赫終於在 1883 年捕獲了霍亂菌。斯諾僅憑模式就推到了同樣的結論——正如孟德爾從清點豌豆，推斷出他看不見的基因。他的地圖開啟的那條線索，一直通到今天的疾病偵探：面對一場新的疫情，他們依舊用清點、繪圖、順水追查來應對。

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