Immunization against viral infections. Successful, ongoing mode of defense.

The body protects itself from disease through a variety of innate defense mechanisms and through immunity acquired both passively and actively. Passive immunization is achieved through transfer of antibody or of sensitized lymphocytes or their soluble products from an immune donor, while active immunization is achieved through stimulation of the recipient's own protective mechanism by means of exposure to the disease or to pathogens or their toxoids. Among the diseases brought under control or eradicated by means of active immunization are smallpox, polio, and measles. Vaccines for immunization against several other diseases are already being used successfully and further ones are being developed.