What is the virus that causes flu? Why doesn't the body create permanent immunity against that virus? How does the vaccine against flu work?

Flu is a disease caused by the influenza virus, a highly mutant DNA virus. Because of the high mutation rate of the virus, that forms many dissimilar strains, flu always presents epidemic features in affected populations and people may have some flu episodes during life (the immune response made from previous infections is not efficient in future infections).

The vaccine against flu is a vaccine made of attenuated virus of three dissimilar strains. Each year the WHO (World Health Organization) researches and determines which are the strains that should compose the vaccine. This is a method to face the high mutation rate of the virus.