When flu strikes, why are some family members reduced to shivering wrecks under their duvets, while others get off with little more than a snuffle?
当流感来袭时，为什么有些人蜷缩在羽绒被下瑟瑟发抖，而有的人最多就是有点鼻塞呢？
Scientists now have an answer, showing that the generation you belong to - and even the year of your birth - predicts how vulnerable you will be to a given strain of seasonal virus.
科学家们近日给出的答案显示，人们出生的年代甚至是年份预示着其抵御特定种类季节性流感的能力。
The flu virus a person first encounters as a child, they found, leaves a permanent "imprint" on the immune system, giving them robust protection against similar strains and much weaker protection against less closely related varieties of the illness.
他们发现，儿时感染的流感病毒会在一个人的免疫系统上留下永久“印记”，人们由此获得对类似疾病的强大免疫力，而对不太相似的疾病的抵抗力就弱得多。
Michael Worobey, head Ecology and Evolutionary Biology at the University of Arizona and a senior author of the study, said: "It's not the age, it's the birth year that matters."
美国亚利桑那大学生态与进化生物学专家、该研究报告的资深作者迈克尔•伍罗贝称，“这不是年龄的问题，而是和出生年份有关。”
In future, seasonal vaccines could be targeted at people of particular ages who are most likely to benefit, and in pandemics when medications are scarce, vulnerable age ranges could be prioritised for protective measures.
未来，可以根据特定年龄区分季节性流感疫苗最能受益的人群。当流感大爆发、药物稀缺时，可以对抵抗力低的年龄群体采取优先保护措施。
"It's breaking new ground for flu, where predictions are really hard," said Worobey. "For any given potential pandemic virus, we can actually now say ... this is the age group that you can expect is going to end up in hospital dying and this is the age group who will be protected."
伍罗贝说，“流感疾病难以预测，该研究在这方面取得了新突破。如今我们可以说，针对任何特定的潜在大型流感病毒，你可以预测哪个年龄群体很可能不治身亡，哪个年龄群体将得到保护。”
The age effect is seen because influenza A viruses - the kind considered most likely to cause pandemics - have evolved into two major branches known as type 1 and type 2 flus.
年龄效应可见于甲型流感病毒中。这是一种最可能广泛传播的流感病毒，已经演变出甲1型流感病毒、甲2型流感病毒两个主要分支。
Up until 1968 all viruses in circulation belonged to the type 1 branch; between 1968 and 1979, type 2 dominated. Since then, strains belonging to both branches have been in circulation simultaneously, but with one type tending to dominate each year. "It's like an oak tree that has a trunk that splits into two major branches and 1968 becomes this really clear dividing line," said Worobey.
1986年以前传播的所有流感病毒都属于甲1型，1968年至1979年间主要传播的是甲2流感病毒。此后，两种类型的流感病毒同时传播，但是每年有一种类型会占据主导地位。“这就像是橡树的主干长出两个主要分枝，1968年是清晰的分界线。”
Using vast databases of historical epidemiological data, the scientists tracked the susceptibility of each birth year from 1918 to the present to the different flus in circulation during their lifetime.
科学家们运用流行病学历史数据的庞大数据库，追踪从1918年至今每一年出生的人群对一生中所遇到的不同流感的易感程度。
The findings, published in the journal Science, showed that the strains in circulation early in life - most people have had flu by the age of five - have a profound impact on which types of flu they would be more sensitive to in the future.
该研究发表在《科学》杂志上，结果显示（大多数人五岁前得过流感）在人们儿时传播的流感很大程度影响了他们未来对哪类流感更为敏感。
"The (first virus you encounter) seems to set you up for life to be quite good at protecting yourself, not just against that particular virus, but also close cousins of that virus," said Worobey.
伍罗贝称，“你感染的第一种流感病毒似乎不仅能使你对其终身免疫，也能抵御其他类似病毒。”
The scientists also studied two bird flu viruses, H5N1 (belonging to the type 1 branch) and H7N9 (type 2), each of which already has caused hundreds of cases of severe illness or death in humans. Scientists are concerned that in the future either of these strains could gain mutations that allow them to not only jump from birds into humans, but also spread rapidly between people, triggering a deadly pandemic.
科学家们对H5N1型（属于1型分支）和H7N9型（2型）禽流感病毒进行了研究，目前已有数百人因此患重病或死亡。科学家们担心，这类病菌未来可能发生突变，不仅能通过禽类传染给人类还可在人类之间迅速传播，导致致命流感大面积传播。
They found that if a person's first flu infection belonged to the same branch as the avian strain, they had a 75% reduced chance of hospitalisation and about an 85% smaller chance of dying.
研究发现，如果一个人首次感染的流感病毒与禽流感病毒同属一个分支，那么他们住院治疗的几率能减少75%，且死亡几率减少85%。
The findings explain the surprising observation that H5N1 tends to cause far more mortalities in younger people - contrary to what might be expected, while H7N9 tends to be most devastating in the elderly population.
研究结果说明，H5N1病毒在年轻人中的致死率更高，这一惊人发现与人们的预测恰恰相反，而H7N9病毒通常对老年人杀伤力更强。
"We're not a completely blank slate when it comes to how susceptible we are to these emerging flu viruses," said Worobey. "Even if we've never been exposed to H5 or H7 viruses, we have some kick-ass protection against one or the other."
“我们对这些新兴流感病毒并非完全没有抵抗力。即使从没有接触过H5或H7型流感病毒，我们也对其中一种有强大的抵抗力。”
This happens, he said, because when a person is infected by flu, their original antibodies are woken up as the body's first line of defence, meaning if they are poorly-matched they will be less efficient at attacking the virus.
他说，之所以会这样是因为当人们感染流感时，其原始抗体作为身体的第一道防线会被唤醒，这意味着如果抗体与病毒不匹配，那么对病毒的攻击便不太有效。
Previously, people had put these patterns down to younger people being more likely to encounter birds, or the virus being more aggressive in elderly people.
过去人们认为，发生上述模式是因为年轻人更容易感染禽流感，或禽流病毒对老年人的攻击力更强。
The scientists said it was not yet clear whether current childhood flu vaccines - which tend to involve components from both branches - would mean in future children would have higher protection against flus of both types.
科学家们称，尚不清楚当下常包含两种流感病毒免疫成分的儿童流感疫苗是否意味着孩子们将来对两种类型的流感具有较强的抵抗力。
Vocabulary
pandemic:(疾病)流行性的；传染于(全国、全洲、全世界)的
susceptible: 易受感染的