Type A influenza viruses are highly unstable. Each year the virus experienced minor changes in antigen structure, called antigenic drift or adaptive mutation.
Can also through antigenic shift mutations or mutations with reasortment process of two or more kinds of antigens. For example, when the Asian flu, H3N2 virus is the result of H1N1 and H2N2 viruses marriage.
Third seasonal H1N1 viruses, H2N2 and H3N2 in humans living can easily live on a pig. Pigs have the privilege of being the place of marriage different viruses and form a new virus.
Pigs have receptors in both animals and humans. Therefore, swine flu virus found in Mexico and type A subtype H1N1, H1N2, H3N2 and H3N1. These viruses derived from pigs that have been reasortment form a new virus and is transmitted to humans.
At least there are about 16 types of hemagglutinin and nine neuraminidase types so you can imagine the number of combinations that could threaten human HN.
Seasonal influenza take place regularly and the WHO estimates that annually kills about 250 thousand to 500 thousand people every year.