Few stories are as prominent in the study of infectious diseases as that of Mary Mallon, a cook to wealthy families, and also to a maternity hospital, in New York in the early 1900s.
infectious disease 传染病
maternity n.母性 maternity hospital 产科医院
As she went from one employer to another, typhoid fever, then deadly in one case in ten, followed in her wake.Public-health officials eventually joined the dots（按点连线/进行调查） and identified her as a carrier of Salmonella typhi（伤寒杆菌）, the bacterium that causes the disease.
typhoid adj.伤寒的 （typhoid mary 伤寒玛丽；坏事传播者）n.伤寒
What was striking about Typhoid Mary, as the newspapers nicknamed her, was that she herself was healthy- proof that people could harbour and transmit S. typhi without showing symptoms of the illness it causes.
harbour n.海港 port；避难所 v.庇护；藏匿；入港停泊
Such silent transmission, as epidemiologists call the phenomenon, has since been observed in many diseases-among them measles（麻疹）, influenza（流感） and HIV/AIDS. A fresh addition to the list is SARS-COV-2, the coronavirus（冠状病毒） behind the covid-19 pandemic now raging.
silent transmission 寂静传染
epidemiologist n.流行病学家
pandemic adj.普遍的，全世界的；（疾病）大流行的 pandemic influenza 大流行性流感
Accumulating evidence suggests a substantial chunk of the infections it causes are transmitted by people whose symptoms have not yet appeared-or even, like Mallon, who never develop symptoms at all.
That has implications for the methods countries are employing to curb the pandemic. Currently, none of the evidence on asymptomatic transmission is watertight.
implication n.暗示；牵连，卷入；含义
asymptomatic adj.无临床症状的
watertight adj.不漏水的，无懈可击的
According to Gerardo Chowell of Georgia State University, in Atlanta, the best way to determine the share（比例） of SARS-COV-2 infections that happen in this way is to follow up a large number of households in which someone is already infected and then track who subsequently infects whom. For this to work, everyone involved would have to be tested daily.
If this were done, comparing subtle variations from person to person in the virus's genetic material would show who caught it from whom. Definitive studies of this nature are not yet available, though some are probably in the works, Dr Chowell reckons.
In the meantime, a growing collection of other research is shedding light on the matter. This work comes in three strands. The first is a set of studies of people in groups for which unusual circumstances have made possible tallying each and every infection.
shed light on 阐明；
使理解
strand n.海滨；线；串 ；（完整故事、主题、情况的）部分 v.（使）搁浅；弄断；（使）落后
tally n.记录；计算 （tally up 总结；结算）；一致，符合（with） n.标签；记账；计数器
These studies permit a fairly precise estimate of the share of those infected who have no symptoms. 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.
bungle v.搞砸（=mess up），笨手笨脚地做 n.拙劣的工作
quarantine n.检疫期 vt.对……进行检疫隔离（~ inspection）
Of 634 people thus infected, 52% had no symptoms at the time of testing, including 18% who never developed symptoms. The residents of Vo, an Italian town in which all 3,300 people were tested twice, is another much-cited example. Of those in Vo found to be infected, 50-75% had no symptoms at the time of the test.
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