One common type of vaccine is a "live vaccine." This type of vaccine contains a small amount of a live virus or bacteria. Before the vaccine is given, scientists weaken the virus or bacteria so it cannot make a person sick. When a person gets a live vaccine, their immune system learns to recognize and fight off that virus or bacteria. Then, if the person is exposed to the virus or bacteria in the future, their immune system will already "know" how to fight it off. Examples of live vaccines include vaccines for measles, mumps, and chickenpox.