While we often refer to it as "the flu," the influenza virus has several strains, with flu A and flu B considered the most common and the most severe.

While we often refer to it as "the flu," the influenza virus has several strains, with flu A and flu B considered the most common and the most severe.

Flu A and flu B are the most common strains of the flu that circulate in humans. The U.S. is currently in the middle of flu season, with a high number of cases reported across the country. There are some key differences between these strains.