The flu is spreading far and wide across the U.S. – and influenza A is largely to blame. Of all the flu tests reported to the Centers for Disease Control and Prevention since the start of flu season last fall,

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.