What are the different types of influenza viruses?

Influenza viruses are classified into four main types: A, B, C, and D. Influenza A viruses are the most common and can infect both humans and animals, often causing seasonal epidemics. Influenza B viruses primarily affect humans and are generally less severe than A viruses, with limited geographical spread. Influenza C causes mild respiratory illness and is not associated with epidemics, while Influenza D primarily affects cattle and is not known to infect humans.