城市型黄热病：如果受感染的人把病毒带入人口稠密的地区，而这些地区有很多人缺乏免疫力，并有伊蚊生存繁殖，就会发生大流行。

Urban yellow fever: Large epidemics occur when infected people introduce the virus into densely populated areas with a high number of non-immune people and Aedes mosquitoes.

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由于人们缺乏对新毒株的免疫力，流感大流行会对包括年轻人群在内的更广泛人群产生甚至更大的影响3,4。

Influenza pandemics can have an even bigger impact in the wider population, including younger age groups, because of a lack of population immunity to the new strain.

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因为大多数人都没有经常和猪接触，所以世界上的许多人都对此病毒缺乏免疫力。

Because most people do not have regular contact with pigs, much of the world's population may lack immunity against swine flu.

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