Aside from other humans, the strongest challengers to human beings are microscopic life forms and viruses. Public health specialists provide preventive measures by immunizing the masses and educating the public about health practices and lifestyle choices most likely to prevent disease and chronic illness. This chronology tracks the development of public health in the United States. It begins in 1796 with the vaccination for smallpox by Dr. Edward Jenner in England, soon adapted as the first public health initiative in America. It follows the course of public health through the development of the germ theory of disease and the subsequent focus on vaccines and antibiotics; the advent of penicillin in the 1940s and the Salk vaccine against polio in the early 1950s; and the gradual shift of public health efforts from combatting infectious diseases to understanding the diseases of aging. The work also includes extensive information on the immune system, along with data on death rates and life expectancies that provide the best measure of the success of public health in the United States.