The sixteenth century in the Americas
In 1520 smallpox reached the Valley of Mexico during the wars around Tenochtitlan. A true virgin‑soil epidemic, it killed leaders and laborers alike, undermining food production and logistics, and contributing to the fall of the capital. Over the next decades the virus traveled the major corridors of people and trade: Caribbean ports to the mainland, Andes mining routes, river valleys and mission circuits. Recurrent waves produced enormous mortality in Mesoamerica and the Andes and, later, across North America, often combining with measles and influenza. Survivors gained immunity, but new births and movements of unexposed people kept the disease in circulation.
Before vaccination: variolation spreads to Europe and the Atlantic world
Long before the eighteenth century, physicians and healers in China, South Asia and parts of Africa practiced variolation—deliberate exposure to small amounts of smallpox material to provoke a milder, protective infection. Methods varied: nasal insufflation of dried scabs, small scratches on the skin, or tiny grafts. Risks were real, but mortality was far lower than in natural epidemics.
Knowledge crossed into the Ottoman Empire and then to Britain in the early 1700s. In 1721, during a Boston outbreak, advocates including Cotton Mather and Zabdiel Boylston promoted variolation despite fierce opposition. By mid‑century, European armies and some American communities used it selectively, especially before campaigns, to avoid catastrophic outbreaks.
Vaccination and the cowpox idea
In 1796 Edward Jenner tested a safer method: vaccinating with cowpox, a related virus that conferred protection against smallpox without causing full disease. He published his findings in 1798. Vaccination spread rapidly because it was safer than variolation and did not itself seed new smallpox outbreaks. Arm‑to‑arm transfer of vaccine lymph kept chains alive before refrigeration.
Governments moved quickly. The Spanish Balmis Expedition (1803–1806) carried vaccine across the Atlantic and Pacific by inoculating a relay of orphan boys. Napoleon vaccinated his armies. Municipal schemes and national laws followed through the nineteenth century, along with vocal anti‑vaccination movements that objected on medical, religious and civil‑liberties grounds.
The nineteenth and twentieth centuries: science, policy and persistence
Industrial cities with dense populations saw periodic surges even after vaccination, often when coverage slipped or vaccine quality was poor. Advances such as glycerinated vaccine, later freeze‑drying, and standardized production improved reliability. Clinicians distinguished two main clinical forms: variola major, with higher fatality, and variola minor (alastrim), usually milder but still dangerous on a population scale.
By the mid‑twentieth century, routine childhood vaccination eliminated smallpox in many countries but left large reservoirs elsewhere. International travel allowed occasional reintroductions. A first WHO eradication effort in 1959 faltered for lack of resources and coordination. A renewed, intensified programme began in 1967 with better tools and strategy.
How smallpox was eradicated
The successful strategy combined strong surveillance with targeted response. Instead of trying to vaccinate everyone, teams hunted cases, traced contacts and built rings of immunity around each outbreak. Two practical innovations made this possible worldwide: a potent, heat‑stable freeze‑dried vaccine and the simple bifurcated needle that let minimally trained workers vaccinate quickly with tiny doses.
By the mid‑1970s transmission was crushed to a few hot spots. The last known naturally occurring case of variola major was recorded in 1975 in Bangladesh; the last known naturally occurring case of any smallpox, a variola minor infection, occurred in Somalia in 1977. A laboratory‑acquired infection in Birmingham, UK, caused one death in 1978. In May 1980, the World Health Assembly declared smallpox eradicated worldwide.
After eradication
Official variola virus stocks remain at two high‑security laboratories, one in the United States and one in Russia, under international oversight. Routine civilian vaccination ended in most countries in the 1970s, leaving younger generations without smallpox immunity. Scientists continue to monitor other orthopoxviruses and maintain vaccines and antivirals as a strategic reserve. Smallpox remains the only human infectious disease eradicated by deliberate action.
Why this history matters
The smallpox story shows how biology, technology and politics interlock. A pathogen that once reshaped empires was contained first by community knowledge (variolation), then by a safer scientific innovation (vaccination), and finally by coordinated public‑health logistics on a global scale. It is a template for what is possible—and a reminder that success depends on coverage, trust and the patient work of surveillance.
