The various types of influenza virus are clearly illustrated in the above figure. Solid squares show the appearance of a new strain causing recurring influenza pandemics. The dotted line indicates the unidentified strains.

The H1N1 form of swine flu is one of the descendants of the Spanish flu that caused a devastating pandemic in humans during 1918 -1919. It would have been persisting in pigs and was then circulated into humans during the 20th century, contributing to the normal seasonal epidemics of influenza.

This virus constantly changes its form, thereby eluding the protective antibodies that people may have developed in response to previous exposures to influenza vaccines. Every two or three years, the virus undergoes minor changes.

But at intervals, a bulk of world’s population has developed some level of resistance to these minor changes and it easily infect populations around the world, often infecting hundreds of millions of people whose antibody defenses are unable to resist it.

In 1957, an Asian flu pandemic infected 45 million Americans and killed 70,000. It caused about two million deaths globally. Once again, the cycle turns back in 2009. So far, 300 peoples in Mexico have been infected by the influenza A swine strain. Thus making evident, the outbreak of swine flu and making contingency plans for a possible global pandemic.