The flu is a disease caused by the influenza
virus,a highly mutant DNA virus.
Due to the high mutation rate of the virus, which forms many
different strains, the flu always presents epidemic features in
affected populations and people may have several flu episodes
during their life.
This is a strategy to combat the high mutation rate of the
virus.The immune response producing during previous infections is
not effective in future infections.
The vaccine against the flu is a vaccine made of the
attenuated virus of three different strains.Each year, the
WHO (World Health Organization) researches and determines which
strains should compose the vaccine.