Flu virus Influenza prevention

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Influenza flu virus ,Flu season, Types of influenza, Prevention Treatment & flu symptoms.

EPIDEMIOLOGY

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.

Every now and then (10 - 15 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 the 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.
See chart below indicating main pandemic strains in previous years.

Influenza A Evolution: 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.

Schools, Creches and places where people are crowded together are the main places where the influenza strains are spread rapidly, especially in children. Influenza epidemics can cause economically significant absenteeism.