There are four main types of influenza (flu) viruses: Influenza A, B, C, and D. Influenza A and B viruses are the primary types that cause seasonal flu epidemics in humans, particularly observed every winter. Influenza A viruses are the only type of influenza virus known to cause global flu pandemics. Influenza C virus infections typically lead to mild symptoms and are not thought to cause epidemics in humans. Influenza D viruses primarily infect animals, mainly cattle; there is currently no evidence that they cause illness in humans.