Antigenic drift

Antigens are found on the surface of viruses and bacteria and are what the body's immune system uses to recognize and fight a particular virus or bacteria. Antigenic drift is when small changes occur to the antigens on the virus or bacteria over time. These changes can be caused by random mutations and happen very slowly. The body’s immune system needs time to recognize and fight the new version of the virus or bacteria, which can cause an outbreak. This is why the flu shot given each year contains small changes to the antigens in it to keep up with the antigenic drift.