Cause
The first flu virus was identified in the 1930s. Since then, scientists have classified influenza viruses into types A, B, and C.
- Type A is the most common and usually causes the most serious epidemics.
- Type B outbreaks also can cause epidemics, but the disease it produces generally is milder than that caused by type A.
- Type C viruses, on the other hand, never have been connected with a large epidemic.