Source for influenza pandemics

There are three ways how influenza A viruses can escape the immune response in the human population: (1) By antigenic drift. This means by mutation and selection of variants under the selection pressure of the immune system. These variants have amino acid replacements mainly in the epitopes of the hemagglutinin. (2) By antigenic shift. This means replacement of at least the hemagglutinin gene of the prevailing human strain by the allelic gene of an avian influenza virus by reassortment. (3) As a rare event, direct or indirect introduction of an avian influenza virus in toto into the human population. A prior introduction of an avian virus into pigs and an adaptation to the new host might be a presupposition for its final passage to humans. In this sense the nowadays situation is reminiscent to that of about 100 years ago, when an avian virus was presumably first introduced into pigs, and from there into humans. Immediately or some time thereafter the disastrous Spanish Flu in 1918/19 had killed at least 20,000,000 people in one winter. Pandemic strains can be created by all three means, however the most common way is by reassortment. In order to recognize a pandemic strain as soon as possible a worldwide surveillance system and collaborating laboratories equipped with corresponding modern technologies are required.