The history of vaccination is, in miniature, the history of medicine's relationship with the public. Edward Jenner's observation in the 1790s that milkmaids who contracted cowpox seemed protected from smallpox led to the first vaccine. The logic of the intervention — introducing a mild related pathogen to generate immunity against a more dangerous one — was sound but operated in advance of any understanding of how the immune system worked. Vaccines existed before immunology did.
The elimination of smallpox through the global vaccination campaign completed in 1980 remains the only instance of the complete eradication of a human infectious disease. Polio is close to eradication, with only two countries — Pakistan and Afghanistan — reporting endemic transmission as of 2024. The success of vaccination programmes depends critically on the concept of herd immunity: when a sufficient proportion of a population is immune, the pathogen cannot find enough susceptible hosts to sustain transmission, and even unvaccinated individuals are protected.
The threshold for herd immunity varies by pathogen — measles, one of the most contagious diseases known, requires approximately 95 percent of the population to be immune to interrupt transmission. For influenza, the threshold is considerably lower. When vaccination coverage falls below the herd immunity threshold, outbreaks become possible even in populations that were previously protected.
