 [ Symptoms ] i read that people get flu shots annually because flu strains change do strains of other diseases change [ Disease ] 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