“Flu viruses are divided into four types: A, B, C, and D. During flu season (October through May), you’ll mostly hear about flu A and flu B. Flu C viruses only cause mild illness in humans and are not linked to large outbreaks. Flu D viruses do not appear to infect humans at all and are mainly found in cattle.

Now that we know there are two main types of flu viruses that affect humans, you may be wondering: Which flu is worse, A or B? Answering that question starts with learning more about their differences and how they affect us.

While flu A and flu B both cause seasonal flu, there are several differences in terms of their structure, behavior, and who they impact.

Flu A is the more common of the two main flu types, making up about 75% of all cases.”

National Council on Aging – NCOA (https://www.ncoa.org/article/whats-the-difference-between-flu-a-and-flu-b/)