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!
i read that people get flu shots annually because flu strains change. do strains of other diseases change?