Smallpox (20th century)

Smallpox stands alone in human history as the only infectious disease that humanity has successfully eradicated — and as one of the deadliest killers our species ever faced. Caused by the Variola virus, smallpox infected humans for at least 3,000 years, with evidence of characteristic pustules found on the mummified remains of Egyptian pharaohs. In the 20th century alone — after vaccine technology existed — it killed an estimated 300 to 500 million people, more than all of the century's wars combined.

The Variola virus spread through respiratory droplets and direct contact with skin lesions. Unlike many pathogens, it infected only humans and had no animal reservoir, a biological fact that ultimately made eradication possible. The disease began with fever, headache, and severe back pain before the defining rash appeared — first flat red spots that progressed to raised bumps, then fluid-filled blisters, then pustules that covered the entire body including the palms, soles, and inside the mouth and throat. The pustular stage was agonizing; survivors were often left permanently scarred, and roughly one-third of those who survived were blinded. Overall case fatality rates averaged 20–30% in unvaccinated populations, rising to 80% or higher in some indigenous American populations encountering the disease for the first time after European contact.

Smallpox was the greatest biological weapon of colonialism. When Spanish conquistadors arrived in the Americas in the early 16th century, they carried Variola to populations with zero immunity. In Mexico, the Aztec population fell from an estimated 25 million to under 2 million within a century, with smallpox doing most of the killing. Similar devastation occurred across North and South America, the Pacific Islands, and Australia when European carriers made contact with isolated populations.

In Europe and Asia, smallpox was endemic — meaning it circulated constantly, killing primarily children. European royalty was not exempt: Queen Mary II of England, Emperor Joseph I of Austria, and Tsar Peter II of Russia all died of smallpox. The search for protection was urgent. In 1796, the English physician Edward Jenner demonstrated that inoculation with the milder cowpox virus conferred immunity against smallpox — the world's first vaccine. Vaccination campaigns gradually reduced smallpox deaths in industrialized nations through the 19th and early 20th centuries, but the disease continued raging in Asia, Africa, and South America.

The decisive moment came in 1967, when the World Health Organization launched an unprecedented global eradication campaign. The strategy combined mass vaccination with aggressive case detection and contact tracing — a concept called "surveillance and containment" that would later be applied to Ebola outbreaks. The last naturally occurring case of smallpox anywhere in the world was Ali Maow Maalin, a hospital cook in Merka, Somalia, in October 1977. On May 8, 1980, the World Health Assembly formally declared smallpox eradicated — the first and still only human infectious disease to achieve that status.

Today, live Variola virus exists in only two officially sanctioned repositories: the CDC in Atlanta and the VECTOR Institute in Russia. The ongoing debate about whether these stocks should be destroyed reflects the unique position smallpox holds in history — simultaneously humanity's greatest viral enemy and potentially its most useful research tool for understanding poxvirus biology and preparing for future threats.