What’s the Difference Between Influenza A and Influenza B — and Does It Matter?

There are some things you can count on as fall turns to winter. The days get shorter. There’s a chill in the air. And chances are good that a respiratory flu virus is beginning to spread.AdvertisementCleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. PolicyFlu season in the United States typically starts in October and tapers off in late April. And there are two main strains of the influenza virus that cause our annual flu season: influenza A and influenza B.If flu acted like a two-person relay race, you’d typically see influenza A at the starting block in autumn, and hitting its full stride by February. Come March, influenza B picks up the viral baton.That said, the flu season can vary from year to year. For example, data from the U.S. Centers for Disease Control and Prevention (CDC) show the 2024-2025 flu season got a late start. The number of reported cases didn’t s...