Antigenic Drift and Shift

Influenza virus is one of the most changeable of viruses. These changes maybe small or large and are abrupt.
Small changes happen in type A and B influenza as the virus makes copies of itself. This process is known as antignic drift. The drifting is frequent enough to make strains of virus the human immune system cant recognize. That is the reason new flu vaccines have to be produced each year to combat that years new strain. Type A influenza also undergoes sudden changes, know as antigenic shift. This shift occurs when two different flu strains infect the same cell and exchange genetic mateial. The assorment of HA or NA proteins in a shifted virus creates a new influenza A subtype. Because people have no immune to new subtype, their appearance tends to coincide with severe flu epidemic or pandemic.