Avaccine is a substance that contains all or part of an infectious version of a disease-causing organism. when the vaccine is administered to a person, the person's immune system attacks the noninfectious version of the organism and learns to recognize its surface proteins. the next time the immune system comes in contact with the same surface proteins, it has a defense already prepared in order to respond quickly to the invading organism. in this way, a vaccine gives people immunity to the disease-causing organism. vaccines can be life-saving, but there have been a few cases of people catching a disease from the administered vaccine.