Types Of Flu »

Influenza A

Influenza A Influenza A is the most common from of flu and usually causes the most serious epidemics. Influenza A virus is essentially an avian virus that has "recently" crossed into mammals. Birds have the greatest number and range of influenza strains. Avian haemagglutinins sometimes appear in pig human and horse influenza strains.

After a gap of ten to fifteen years, a major new pandemic strain appears in man, with a totally new HA and sometimes a new NA as well (antigenic shift). This variant causes a major epidemic around the world (pandemic). Over a period of subsequent years, this strain undergoes minor changes (antigenic drift) every two to three years, probably driven by selective antibody pressure in the populations of humans infected.

Evolution of Influenza A

1874 --- (H3N8)

1890 --- (H2N2) .........................Pandemic

1902 --- (H3N2)

1918 --- (H1N1)..........................Pandemic

1933 --- (H1N1)..........................First strains isolated

1947 --- (H1N1)..........................Variation detected

1957 --- (H2N2)..........................Asian Flu pandemic

1968 --- (H3N2).........................."Hong Kong " Flu pandemic

1976 --- (H1N1)..........................Swine Flu , non-epidemic

1977 --- (H1N1) + (H3N2)............Russian Flu epidemic

This constant antigenic change down the years means that new vaccines have to be made on a regular basis to overcome this grave health problem. The H5N1 strain is the latest form of Influenza A virus now on the verge of causing a flu pandemic.