This timeline details pandemics through history
A pandemic is defined as a disease that spreads across a wide geographical area. Pandemics are generally caused by a new virus, or by a strain of an existing virus that spreads rapidly. They may also be caused by bacteria, though this is rarer nowadays unless it is resistant to antibiotics.
It was originally thought that the disease was spread by black rats, but due to the rate of spread and the lack of evidence of large numbers of rat corpses (Bubonic plague kills rats), it is now thought that the bacillus that caused the disease was airborne; spread by the victims themselves.