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.
The virus then spread to Alaska and Canada. From there it spread to the continental U.S. and western Europe. From Europe, the flu spread to the Southern Hemisphere. Because global travel is now so easy, many scientists fear that the flu will affect 40 percent of the world s population. Pandemics are characterized by antigenic shifts and are associated with severe illness and significant mortality on a global scale.