How the flu works
The influenza virus infects a staggering 5 million people worldwide every single year, travelling from person to person in airborne droplets, and causing chills, fever, sore throat, runny nose, headaches and muscle pain.
The flu virus changes gradually by a process known as antigenic drift. As the virus replicates, single nucleotide errors occur in the viral genome, causing minute changes to the proteins that coat the outside of the virus. The immune system recognises these proteins to detect and destroy the infection, so as they change, the ability of the body to recognise the virus decreases, preventing people from building up immunity.
Not only does the virus make continual, subtle changes to its genome and proteins, but it also occasionally develops huge mutations.