When I was a child, I noticed a strange scar on my mother’s upper arm—a circle of small indents around a larger one. It fascinated me for a while, but like many childhood curiosities, I eventually forgot about it. Years later, seeing the same scar on an elderly woman’s arm reignited my curiosity and led me to finally ask my mother about it.

She reminded me that I already knew the answer: the scar came from the smallpox vaccine. Smallpox was a highly contagious and often deadly disease caused by the variola virus, marked by fever and a severe rash that left many survivors scarred. For centuries, it devastated populations until a global vaccination effort finally brought it under control.

The smallpox vaccine was administered using a two-pronged needle that punctured the skin multiple times, introducing a live but less harmful virus called vaccinia. The body’s reaction caused a blister that eventually scabbed over and healed into a permanent scar. That mark became a visible sign that someone had been protected against smallpox.

Thanks to widespread vaccination, smallpox was declared eradicated worldwide in 1980, and routine vaccinations ended in the U.S. in the early 1970s. Today, the smallpox scar is a reminder of one of humanity’s greatest public health achievements—a lasting symbol of how global cooperation and vaccines eliminated a deadly disease.