How many different flu viruses have been identified so far?

As of now, four main types of influenza viruses have been identified: influenza A, B, C, and D. Influenza A and B are responsible for seasonal flu epidemics, with numerous subtypes and strains within them. Influenza C causes milder respiratory symptoms and is less common, while influenza D primarily affects cattle and is not known to infect humans. The exact number of different strains can vary significantly each year due to mutations and reassortment, particularly in influenza A viruses.