Four types of influenza viruses cause influenza. These are influenza A, B, C, and D viruses. Influenza A and B viruses cause seasonal epidemics of illness in humans, particularly each winter.

Influenza A viruses are considered the only influenza viruses known to cause influenza pandemics. Influenza C virus infections cause mild illness and are not thought to cause human pandemics.

Influenza D viruses primarily affect cattle, infecting other animals, but are not known to infect humans in a way that causes illness.