The first flu virus was identified in the 1930s. Unlike other diseases, there are slight changes in the flu virus antigens, called antigen drifts, and major changes called antigen shifts. There are three types of the flu-A, B, and C. Type A is the most prevalent and is associated with the most serious epidemics. Type B can reach epidemic levels, but is generally milder. Type C has never been connected to large epidemics.
Scientists are unable to control the flu because the virus is constantly mutating.
As a result of these mutations, new vaccines must be developed every year. New strains of the flu emerge approximately every 100 years. The current flu virus, the Sydney virus, is a mutation of the Hong Kong Flu. Major changes in the flu virus result in worldwide epidemics. Scientists believe flu outbreaks began in China where humans and animals live in close contact.