Flu A vs. Flu B: Understanding the Distinctions Between the Two Main Influenza Strains

Understand the differences between Influenza A and Influenza B, including how they spread, who they affect, and which strain tends to be more severe.

By: AXL Media

Published: Feb 26, 2026, 7:34 AM EST

Source: Health

Influenza A: The Versatile Threat

Influenza A is the most common type of flu and is responsible for the majority of seasonal flu cases and all known flu pandemics.

The Host Range: Unlike other strains, Flu A can infect humans, birds, pigs, horses, and other animals. This "species hopping" allows the virus to mutate rapidly (antigenic shift), creating new strains that the human immune system doesn't recognize.

Subtypes: Flu A is categorized by two proteins on its surface: Hemagglutinin (H) and Neuraminidase (N). Common seasonal subtypes include H1N1 and H3N2.

Severity: Historically, Influenza A (specifically the H3N2 subtype) is associated with more severe illness, higher hospitalization rates, and increased mortality in older adults.

Influenza B: The Human Specialist