The impending elimination of paralytic polio emphasizes how effective vaccines can be in preventing infectious diseases. Live, attenuated viral vaccines offer potent immunity, but the fact that these are live pathogens means that some individuals, usually those with underlying immunocompromise, become ill in response to the vaccine strain of the virus. Provoking both cell-mediated and humoral immunity is another advantage of DNA vaccines. Salmonellae are normally enteric pathogens, initiating infection by invading the epithelial layer of the small intestine. Due to this route of infection, salmonellae are natural candidates for the development of vaccines intended to generate mucosal immunity. A factor that inhibits the effectiveness of antibacterial vaccines is the ability of many bacterial species to vary the antigens they express. It has been proposed that cytotoxic T lymphocytes (CTLs) may be needed for resolution of a primary infection while antiantibody is important for preventing the initiation of a subsequent infection. As many of the currently licensed or available vaccines for viral infections use live, attenuated virus particles, safety is of paramount importance in the development of viral vaccines. The development of vaccines has been, and continues to be, one of the greatest accomplishments in modern medicine. Passive therapy also has shown efficacy in prevention and even treatment of some infectious diseases, and newer technologies involved in production of human monoclonal antibodies likely will increase the range of diseases that are amenable to therapy by passive reagents.