5. The human immune system is a complex network of cells and organs that evolved
to fight off infectious pathogens. Vaccines teach the immune system by mimicking
a natural infection. The influenza vaccine, contains a weakened form of the virus
that doesn't cause severe disease or reproduce very well. Human white blood cells
(WBC's ) can't tell that the vaccine viruses are weakened, so they kill the viruses.
Antibody levels start to decline over time and the influenza virus evolves very
rapidly. Why does and individual need to get a yearly flu vaccine?