More than 95% of cases are self-limited with clearance of virus and immunity to reinfection.

超过95%的病例是自限性清除病毒，并且对再次感染免疫。

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As people would have no natural immunity, a new influenza virus could cause widespread death, illness, social and economic disruption.

由于人无天然的免疫力，一种新的流感病毒可引起广泛的死亡、疾病、社会和经济破坏。

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"We actually control for their immunity," he says. "the explanation isn't that extroverts interact with more people, and therefore have immunity to that virus."

“我们实际上控制了他们的免疫力，”他说，“解释不是性格外向的人与更多的人接触因此而对病毒有免疫力。”

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Newborns need a vaccination within the first day of life to prevent transmission of the virus from their mothers, and babies need two more doses within the first six months for full immunity.

VOA: standard.2010.07.26

Head of the International Red Cross Federation's Influenza team, Robert Kaufman, says the impact of a novel virus on a community without immunity is not just about the number of illnesses and deaths.

VOA: standard.2009.09.04

"To start with, we'll take 15 volunteers, with no immunity to the particular flu virus and infect five of them, " he said.

BBC: Colds and flu remain mysterious

Some believe that it was the constant exposure to the virus which allowed them to develop the immunity, and on stopping work, they became vulnerable.

BBC: HIV vaccine trials 'within months'

When eaten, the virus is taken up in the lymphatic tissue in the throat, and immunity to rabies develops.

FORBES: A Pox On Your House? How Fighting One Disease Brought Back Another