Flu viruses, however, evolve so fast that this "immune memory" provides only partial protection.

然而，流感病毒进化如此之快，以致于这种“免疫记忆”只能提供部分保护。

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We can also say that once most people have been infected by swine flu and become immune, a variant that can beat that immunity will replace the current strain.

我们还可以确定的是，一旦大部分人群感染猪流感并获得免疫力，疾病的变种将会打破目前的免疫性并成为当下的压力。

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The flu has battered down even the hardiest parts of the immune system.

这“流感”甚至已经破坏免疫系统最强大的部分了。

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The end result is a virus with the exterior proteins of the H1N1 swine flu -- so the immune system recognizes it -- but with the inner mechanics -- the fast-growth ability -- of X-157.

CNN: Candidate virus for H1N1 vaccine arrives at CDC

Some doctors paint a scary worst-case scenario: A flu pandemic breaks out and beats down the immune systems of millions of people, and then staphylococci--the hospital strain and the nastier variant in the community--run wild on a killing spree.

FORBES: Germ Warfare

Finally, the flu shot does not always work -- particularly among the elderly and people with weak immune systems.

CNN: Flu shot myths addressed