Cowpox and Smallpox are very similar diseases. When cowpox is injected into the body in a form of a vaccine, a weakened germ cell of the disease enters the body. Since it is weakened, the body is able to easily fight it off by sending antibodies. After the antibodies kill off the germ cell, B memory cells hold the information of the threat, so if in the future, when the host contracts the virus/disease again, the body is able to counteract it faster than it did the first time around. So since Cowpox and Smallpox are very similar, when a person is first injected with the Cowpox vaccine, the B memory cell remembers information about Cowpox. Then when the host contracts Smallpox, the body sees Smallpox as the same thing as Cowpox, so the body can fight off Smallpox quickly, and thus preventing Smallpox.
