The immune system recognizes vaccine agents as foreign, destroys them, and 'remembers' them. When the virulent version of an agent comes along the body recognizes the protein coat on the virus, and thus is prepared to respond, by (1) neutralizing the target agent before it can enter cells, and (2) by recognizing and destroying infected cells before that agent can multiply to vast numbers.

When two or more vaccines are mixed together in the same formulation, the two vaccines can interfere. This most frequently occurs with live attenuated vaccines, where one of the vaccine components is more robust than the others and suppresses the growth and immune response to the other components. This phenomenon was first noted in the trivalent Sabin polio vaccine, where the amount of serotype 2 virus in the vaccine had to be reduced to stop it from interfering with the "take" of the serotype 1 and 2 viruses in the vaccine. 1 This phenomenon has also been found to be a problem with the dengue vaccines currently being researched, where the DEN-3 serotype was found to predominate and suppress the response to DEN-1, -2 and -4 serotypes. 2

Vaccines have contributed to the eradication of smallpox, one of the most contagious and deadly diseases known to man. Other diseases such as rubella, polio, measles, mumps, chickenpox, and typhoid are nowhere near as common as they were a hundred years ago. As long as the vast majority of people are vaccinated, it is much more difficult for an outbreak of disease to occur, let alone spread. This effect is called herd immunity. Polio, which is transmitted only between humans, is targeted by an extensive eradication campaign that has seen endemic polio restricted to only parts of four countries (Afghanistan, India, Nigeria and Pakistan). 3 The difficulty of reaching all children as well as cultural misunderstandings, however, have caused the anticipated eradication date to be missed several times.

1.Sutter RW, Cochi SL, Melnick JL (1999). "Live attenuated polio vaccines". in Plotkin SA, Orenstein WA (eds.). Vaccines. Philadelphia: W. B. Saunders. pp. 364–408.
2.Kanesa-thasan N, Sun W, Kim-Ahn G, et al. (2001). "Safety and immunogenicity of attenuated dengue virus vaccines (Aventis Pasteur) in human volunteers". Vaccine 19 (23–24): 3179–3188. doi:10.1016/S0264-410X(01)00020-2. PMID 11312014.
3.a b c Stern AM, Markel H (2005). "The history of vaccines and immunization: familiar patterns, new challenges". Health Aff 24 (3): 611–21. doi:10.1377/hlthaff.24.3.611. PMID 15886151. http://content.healthaffairs.org/cgi/content/full/24/3/611.