When I was a child, I noticed a strange circular scar on my mother’s upper arm—a ring of tiny indents surrounding a larger one. It fascinated me briefly before fading from memory, as childhood curiosities often do. Years later, while helping an elderly woman off a train, I saw the exact same scar in the same place. With my curiosity suddenly rekindled, I called my mother to ask about it. Her answer was simple: it was from the smallpox vaccine.

Smallpox was once a highly contagious and deadly disease caused by the variola virus, killing about 30% of those infected in the 20th century. Thanks to widespread vaccination efforts, the disease was eradicated globally in 1980, and routine vaccinations in the U.S. ended in the early 1970s. That’s why only older generations still have the trademark scar.

The scar came from the way the smallpox vaccine was administered. Doctors used a two-pronged needle to make several quick punctures in the skin, delivering a live but less harmful virus called vaccinia. The injection site would form a blister, then a scab, and finally heal into the distinctive round scar that became a universal marker of immunity.

Today, that small scar represents one of humanity’s greatest triumphs: the complete eradication of a deadly disease. Younger generations will never need the vaccine, while older ones carry a visible reminder of a public health victory. Do you—or someone you know—have a smallpox vaccine scar? Feel free to share your memories and stories.