(injection of some dead or weakened virus, or bacteria that causes the disease). by doing so, the body learns how the virus/bacteria harms the body, and will react more quickly to fight the virus/bacteria when it comes in contact with the virus/bacteria again. when your body has defended itself against the virus/bacteria it will trap the certain virus/bacteria in a "net" so when the virus/bacteria comes back it will be easier to trap those viruses/bacteria as well.