What is influenza A, the type of flu making people so sick right now?

(NEXSTAR) – 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, about 97% have turned up positive for influenza A.

Influenza A is further divided into subtypes. You may have heard of H1N1 or H3N2 – those are the two dominant types of influenza A circulating right now, according to the CDC.

“Influenza A typically causes worse symptoms compared to influenza B, and patients are more likely to get hospitalized with influenza A compared to influenza B,” said Dr. Donald Dumford, an infectious disease specialist at Cleveland Clinic.

But Dr. Amesh Adalja, an infectious disease expert at Johns Hopkins Center for Health Security, warned influenza B can also be dangerous. “Influenza B can be severe as well, but influenza A is the dominant version of influenza infecting people right now.”

Both types spread the same way, cause similar symptoms, and are most dangerous for babies and the elderly. The flu vaccine is formulated to protect against both types.

The dominance of influenza A at this point in the flu season is to be expected. Influenza A is more commonly seen earlier in flu season, in the winter months, while influenza B seems to crop up later, around early spring.

Dumford expects the shift to happen this year in late February or early March.

There are also two other types of influenza – C and D – but we don’t worry about them as much. Influenza C typically causes mild illness and doesn’t cause widespread epidemics, the CDC says, while influenza D mostly affects cattle.