Vaccines work by simulating the body's immune response to diseases. They contain preparations that mimic viruses or bacteria so that the immune system can make antibodies against them. When a vaccinated person is exposed to the real disease in the future, their immune system will remember how to fight it off. Vaccines are usually administered via needle injections but some can be given orally or nasally. Vaccination is currently the safest way for people to gain immunity against various harmful bacteria and viruses.