The influenza virus is one of the most changeable of viruses. These genetic changes may be small and continuous or large and abrupt. Small, continuous changes happen in type A and type B influenza as the influenza virus makes copies of itself. The process is called antigenic drift. The drifting is frequent enough to make the new strain of influenza virus often unrecognizable to the human immune system. For this reason, a new flu vaccine must be produced each year to combat that year's prevalent strains.
Type A influenza also undergoes infrequent and sudden changes, called antigenic shift. Antigenic shift occurs when two different flu strains infect the same cell and exchange genetic material. The novel assortment of HA or NA proteins in a shifted virus creates a new influenza A subtype. Because people have little or no immunity to such a new subtype, their appearance tends to coincide with severe flu epidemics or pandemics.
(Click Pandemic for more information about pandemics.)