Influenza
Influenza (also known as the flu), is a highly contagious disease of the respiratory tract caused by influenza viruses. There are three types of influenza viruses that infect humans- influenza A, B and C. Influenza A and B are able to cause severe major out brakes and serious disease, however only Influenza A is known to have been responsible for influenza pandemics. Type C is considerably less extreme and is similar to a cold-like illness in children.
If you are infected with an influenza virus your body develops an antibody against that virus however the viruses are continually changing and as a result the antibody you currently have does not work against the modified disease, meaning you become ill.