Flu is highly infectious.

流感的传染性很高。

《牛津词典》

They say the of cases is likely to climb higher before off because flu is highly contagious in victims and spread it even after feeling better.

他们称病症很可能相比以前更快攀升，因为流感在受害者中传染性极强而且传播甚至更为迅速。

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But when it has, the bird flu has proved to be a highly infectious and potent disease, that is fatal 60 percent of the time.

但这种禽流感病毒已经被证明是一种高传染性且强有力的疾病，60%将是致命的。

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Secondly, the 2009 H1N1 lacks a gene that is present in highly virulent flu viruses, such as the one in 1918.

CNN: Swine flu, part II: What's in store in the months to come

Norovirus is highly contagious, even moreso than the flu.

FORBES: Norovirus: The Bug That Is Presently Making So Many Of Us Sick