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

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

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