WebMD Answers
Some viruses tend to adapt and mutate (change), and the influenza virus changes all of the time. Medical science has a full time job keeping up with this primitive adaptation. Bacteria can change, too ... this is why we have drug-resistant strains of Staph (MRSA).
Yes, you have to get a different flu vaccine every year, depending on the strains that are circulating.
People who had pertussis (whooping cough) as a child are not always immune for life -- the recent epidemic proves this. Be glad that we have scientists keeping track of these things!
This answer should not be considered medical advice...
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