When I was a child, I noticed a curious scar on my mother’s upper arm—a small ring of indents encircling a larger one. It fascinated me for a while, but like many childhood mysteries, I soon forgot about it. Years later, while helping an elderly woman off a train, I spotted the same mark in the same place. My curiosity reignited, I asked my mother about it. Her answer was simple yet astonishing: it was the smallpox vaccine scar.

Smallpox was a highly contagious and deadly disease caused by the variola virus. It began with fever and fatigue, followed by a rash that turned into pus-filled sores. Survivors were often left with disfiguring scars, and the disease killed about 30% of those infected. For centuries, smallpox devastated populations across the globe. Thanks to widespread vaccination, however, the World Health Organization declared it eradicated in 1980—one of humanity’s greatest medical triumphs. In the United States, routine vaccinations stopped in 1972 after the disease was eliminated domestically.

The smallpox vaccine left a distinctive scar because of how it was administered. Using a bifurcated, or two-pronged, needle dipped in vaccine solution, health workers pricked the skin several times in quick succession. The vaccine contained vaccinia, a live virus related to smallpox but far less dangerous. After vaccination, the injection site formed a bump, then a blister that scabbed over and healed into the permanent mark we recognize today. This scar became a visible badge of immunity—a symbol of protection in a time when smallpox was still a threat. For younger generations, the absence of that small round scar marks a world free from smallpox, while for those who bear it, the scar is a piece of living history—a reminder of science’s triumph over one of the deadliest diseases in human existence.