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Few stories are as prominent in the study of infectious diseases as that of Mary Mallon,
在傳染病研究領域，很少有像瑪麗·梅倫的故事那樣引人注目。
a cook to wealthy families, and also to a maternity hospital, in New York in the early 1900s.
20世紀初，瑪麗·馬倫在富裕的家庭和產科醫院做過廚師。
As she went from one employer to another, typhoid fever, then deadly in one case in ten, followed in her wake.
她的雇主換了一個有一個，傷寒都緊隨其后，10個人中就有一個因其死亡。
Public-health officials eventually joined the dots and identified her as a carrier of Salmonella typhi, the bacterium that causes the disease.
最終公共衛生部進行調查，并確定她是傷寒桿菌（一種會引起疾病的細菌）的攜帶者。
What was striking about Typhoid Mary, as the newspapers nicknamed her, was that she herself was healthy—
紙媒稱她為傷寒瑪麗 ，關于她的故事最引人注目的是她本身是健康的——
proof that people could harbour and transmit S. typhi without showing symptoms of the illness it causes.
這證明人們會攜帶和傳染傷寒桿菌，且不出現任何癥狀。
Such silent transmission, as epidemiologists call the phenomenon, has since been observed in many diseases—among them measles, influenza and HIV/AIDS.
流行病學家稱這種現象為寂靜傳染，很多疾病，如麻疹、流感HIV和AIDS，都出現過這種現象。
A fresh addition to the list is SARS-COV-2, the coronavirus behind the covid-19 pandemic now raging.
最新進入這份名單的是SARS-COV-2，這是冠狀病毒是目前covid-19疫情肆意的幕后推手。
Accumulating evidence suggests a substantial chunk of the infections it causes
越來越多的證據表明，它引起的感染中有很大一部分
are transmitted by people whose symptoms have not yet appeared—or even, like Mallon, who never develop symptoms at all.
是由尚未出現癥狀的人傳播的，甚至像馬倫一樣，根本沒有出現癥狀。
That has implications for the methods countries are employing to curb the pandemic.
這對各國控制全球流行病的方法有影響。
Currently, none of the evidence on asymptomatic transmission is watertight.
目前，關于無癥狀傳染的證據沒有一個是無懈可擊的。
According to Gerardo Chowell of Georgia State University, in Atlanta,
根據亞特蘭大佐治亞州立大學的Gerardo Chowell，
the best way to determine the share of SARS-COV-2 infections that happen in this way
要確定SARS-COV-2以這種方式傳染的比列
is to follow up a large number of households in which someone is already infected and then track who subsequently infects whom.
最好的方法是對大量已有感染者的家庭進行跟蹤，然后跟蹤誰接著感染了誰。
For this to work, everyone involved would have to be tested daily.
這種方法需要每個人每天接受檢測。
If this were done, comparing subtle variations from person to person in the virus's genetic material would show who caught it from whom.
做完這一步后，比較病毒基因材料中人與之間的細微差異，就可以發現是誰從誰那里感染了病毒。
Definitive studies of this nature are not yet available, though some are probably in the works, Dr Chowell reckons.
Chowell博士認為，目前還沒有關于這一性質的權威研究，盡管一些研究可能正在進行中。
In the meantime, a growing collection of other research is shedding light on the matter. This work comes in three strands.
與此同時，越來越多的其他研究正在闡明這個問題。這項工作分為三個部分。
The first is a set of studies of people in groups for which unusual circumstances have made possible tallying each and every infection.
首先是對人群的一組研究，不同尋常的環境使統計每一個感染患者成為可能。
These studies permit a fairly precise estimate of the share of those infected who have no symptoms.
這些研究允許對沒有癥狀的感染者的比例進行相當精確的估計。
One such group are the passengers and crew of the Diamond Princess, a cruise ship on-board which the infection rate exploded because of a bungled quarantine.
鉆石公主號郵輪上的乘客和船員就是這樣的一群人。由于疏于檢疫，郵輪上的感染率出現了爆炸性增長。
Of 634 people thus infected, 52% had no symptoms at the time of testing, including 18% who never developed symptoms.
634名感染者中，52%的患者在檢測時都沒任何癥狀，其中18%從未出現癥狀。
The residents of Vo, an Italian town in which all 3,300 people were tested twice, is another much-cited example.
另一個被廣泛引用的例子是意大利小城Vo的居民，該市所有3300人都接受了兩次檢測。
Of those in Vo found to be infected, 50-75% had no symptoms at the time of the test.
感染者中，50-75%的人在檢測時沒有出現癥狀。
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