To date, smallpox is one of only two diseases officially eradicated according to the WHO. How did eradication occur, why are immunizations important, and what do cows have to do with smallpox?
The world was declared free of smallpox, an infectious disease caused by the variola virus, in 1980. This continues to be considered one of the greatest international public health achievements in history.
Although the origin of the disease is unknown, rashes similar to those caused by smallpox were found on mummified corpses from the 3rd century in Ancient Egypt. The mummified head of the Egyptian pharaoh Ramses V, who died in 1156 BC, bears the same markings. Further, written descriptions of the highly contagious disease appear in China as early as the 4th century.
As ancient civilizations grew, populations became more mobile and the development of global trade routes encouraged vast exploration. By the 18th century, European colonization had exacerbated the global spread of smallpox to regions including Africa, North and South America, Australia, and the Caribbean. The harsh exposure to this disease on native populations was devastating, impacting the decline of civilizations including the Aztecs and Incas. During the French-Indian War, smallpox-infected blankets were deliberately used by British forces against the native populations of North America.  With no way to protect yourself against the disease, smallpox indiscriminately wreaked havoc wherever it traveled.
On average, THREE in TEN individuals infected with smallpox died. Those infected developed a fever and “disfiguring skin rash” that left one-third of survivors blind and many more with permanent, severe scarring.
The earliest treatments to smallpox were borne from the understanding that survivors became immune to future infection. Doctors and other healers would introduce a portion of a smallpox pustule that came from a survivor into the arm or leg of an unaffected individual. This was called “inoculation” and represents an early understanding of active immunity. Although the technique reduced the death rate associated with smallpox, the disease persisted and not all those who were inoculated were immune.
Here we introduce the hero in this immunization story: Edward Jenner. Although he may not be the most famous Jenner you’ve heard of, he is an influential figure in our public health history. Jenner was inoculated with smallpox as a young boy in England and developed an immunity. Throughout adolescence, he developed an interest in science that motivated him to pursue a career in medicine. While apprenticing a local surgeon, Jenner heard a milkmaid exclaim, “I shall never have smallpox for I have had cowpox.” It was common knowledge, at the time, that milkmaids were protected from the ongoing smallpox pandemic. But why?
Through experimentation, Dr. Jenner recruited milkmaids who had been infected by cowpox, a mild and non-threatening virus transmitted to humans by cows, and inoculated them with smallpox…The result?
The women never developed signs of smallpox! To test the theory further, Dr. Jenner repeatedly inoculated his gardener’s healthy 8-year-old son with material from a local milkmaid’s cowpox sore—the earliest smallpox vaccine (as seen in the oil painting below by Ernest Board). The young boy never developed smallpox and Jenner went on to publish his treatise “On the Origin of Vaccine Inoculation.” In this publication, he explained the protective effects of cowpox against the variola virus that caused smallpox.
Jenner’s work paved the way for smallpox eradication worldwide. Vaccination replaced inoculation and global immunizations programs were funded to ensure herd immunity. This means that enough people around the world were vaccinated against smallpox that the virus was not able to transmit as easily from person to person. Entire communities were less likely to get the disease and those who could not get immunized (those who were immunosuppressed) were protected by the vaccinated individuals surrounding them.  Herd immunity is how we, as a world, wiped out smallpox together. And we could not have done it without effective immunization.
As we near a vaccine to protect against COVID-19, you can learn more about herd immunity and the overwhelmingly beneficial impact of immunizations with the helpful resources below:
 Riedel, S. (2005). Edward Jenner and the History of Smallpox and Vaccination. Baylor University Medical Center Proceedings, 18(1), 21-25. doi:10.1080/08998280.2005.11928028. Retrieved from https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1200696/