Types A and B influenza are the more common forms of this infection, routinely causing seasonal outbreaks. Type C influenza usually only causes mild respiratory infections.
Type B influenza can be just as severe as type A influenza but is less common throughout the flu season compared to type A.
Humans are the natural host for type B infection. Type B viruses mutate much slower than type A infections and are categorized by strains, but not subtypes. The B virus strains take longer for their genetic makeup to change than influenza A. This drastically reduces the risk of a widespread pandemic due to type B influenza.
Type A influenza can be dangerous and is known to cause outbreaks and increase your risk of disease. Unlike a type B infection, type A viruses are categorized by subtypes and strains. Influenza A mutates faster than influenza B, but both viruses are always changing, creating new strains from one flu season to the next. Past flu vaccinations will not prevent infection from a new strain.
Wild birds are the natural hosts for a type A virus, also called ian flu and bird flu. This infection can also spread to other animals and humans. This, combined with the ability of type A influenza to mutate faster than type B, can cause pandemics.