What Causes the Flu? Influenza Viruses The flu is caused by influenza viruses, which are classified into four main types: A, B, C, and D. Influenza A and B are The most common types that cause seasonal epidemics in humans. These viruses mutate frequently, so flu vaccines must be updated yearly. Types of Influenza Viruses … Continue reading Protect Yourself This Flu Season at Health Link Pharmacy!