Thanks to widespread vaccination, smallpox was declared eradicated in the United States in 1952, and routine vaccinations stopped in 1972. Before then, nearly all children were vaccinated—and the vaccine left a permanent mark. That scar became a visible sign that someone had been protected against smallpox.

Unlike modern vaccines, the smallpox vaccine was administered using a two-pronged needle that made multiple punctures in the skin. This caused a localized reaction: bumps formed, turned into fluid-filled blisters, then scabbed over as the skin healed. The result was the familiar round, indented scar many older adults still have today. That’s the scar my mother carries—and why so many people of her generation share the same mark of history.