The reason is that the influenza virus mutates much more rapidly than most other viruses.A person who develops immunity to one strain of the virus is not well protected from a different strain.

• 这是由于流感病毒比大部分病毒的基因变异都要快，一个人即使对一类流感病毒产生了免疫，也未必能抵御另外一种流感病毒。