The biological preparations that stimulate immunity against specific pathogens — from Jenner's smallpox vaccine through the 20th-century programs that eradicated smallpox and nearly eradicated polio, to the mRNA vaccines developed against COVID-19 in less than a year. Vaccines are the most cost-effective public health intervention ever developed, and the politics of their acceptance have become as consequential as the science of their development.
Smallpox killed an estimated 300 million people in the 20th century alone — more than all the wars of that century combined. It no longer exists. On May 8, 1980, the World Health Organization declared smallpox eradicated — the first and still the only time humanity has deliberately driven a human pathogen to extinction. The last natural case occurred in 1977 in Somalia. The instrument of eradication was a vaccine. Edward Jenner developed the first version in 1796, using material from cowpox lesions. The WHO's global eradication campaign ran from 1967 to 1977, vaccinating hundreds of millions of people across dozens of countries. The campaign is the most successful public health program in history, and smallpox eradication is among humanity's greatest collective achievements.