I have a clear memory of noticing a distinct scar on my mother’s arm when I was a child. It sat high near her shoulder, a ring of small indents surrounding a larger one. For some reason, it fascinated me, though I can’t recall why. Over time, I stopped thinking about it and eventually forgot I had ever been curious.

Years later, I was reminded of it while helping an elderly woman off a train. I noticed the exact same type of scar in the same place on her arm. Curious again but unable to ask her, I called my mother. She told me she had already explained it to me before, though I had clearly forgotten. The scar, she said, came from the smallpox vaccine.

Smallpox was once a devastating infectious disease that caused fever, severe rashes, and often death. During major outbreaks, it killed about three out of every ten infected people, and many survivors were left with lasting disfigurement. Until the early 1970s, children were routinely vaccinated, and the vaccine left a visible, permanent scar—proof of immunity.

The scar formed because the vaccine was administered using a two-pronged needle that made multiple punctures in the skin. This caused a localized infection, leading to bumps that turned into fluid-filled blisters. As they healed, they scabbed over and left behind the characteristic circular scar still visible on many people today.