The significance of antigenic shift lies in its potential to cause widespread illness. Because the new viral strain is markedly different from existing strains, the general population often has little to no pre-existing immunity. This lack of immunity can facilitate rapid and extensive transmission, resulting in large-scale outbreaks or even global pandemics. Historical examples include the 1918 Spanish Flu, the 1957 Asian Flu, and the 2009 H1N1 pandemic.
