Sometime during the 1770s Edward Jenner heard a milkmaid boast that she would never have the often-fatal or disfiguring disease smallpox, because she had already had cowpox, which has a very mild effect in humans. In 1796 Jenner took pus from the hand of a milkmaid with cowpox, inoculated an 8-year-old boy with it, then six weeks later variolated (exposed) the boy's arm with smallpox, afterwards observing that the boy did not catch smallpox.1 Since vaccination with cowpox was much safer than smallpox inoculation,2 the latter, though still widely practiced in England, was banned in 1840.3 Louis Pasteur generalized Jenner's idea by developing what he called a rabies vaccine (now termed an antitoxin), and in the 19th century vaccines were considered a matter of national prestige, and compulsory vaccination laws were passed. 1

The 20th century saw the introduction of several successful vaccines, including those against diphtheria, measles, mumps, and rubella. Major achievements included the development of the polio vaccine in the 1950s and the eradication of smallpox during the 1960s and 1970s. As vaccines became more common, many people began taking them for granted. However, vaccines remained elusive for many important diseases, including malaria and HIV. 1

1.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. 
2.Van Sant JE (2008). "The Vaccinators: Smallpox, Medical Knowledge, and the 'Opening' of Japan". J Hist Med Allied Sci 63 (2): 276–9. doi:10.1093/jhmas/jrn014. 
3.Dudgeon JA (1963). "Development of smallpox vaccine in England in the eighteenth and nineteenth centuries". BMJ (5342): 1367–72. doi:10.1136/bmj.1.5342.1367.