Influenza is a contagious respiratory illness caused by influenza viruses that infect the nose, throat, and sometimes the lungs. It can cause mild to severe illness, and at times can lead to death. The best way to prevent flu is by getting a flu vaccine each year.
A pandemic is the rapid spread of a new human influenza around the world. Influenza pandemics happen when a new strain of a flu virus appears which can infect humans, to which most people have no immunity and which can transmit efficiently from human to human.