As a child, I distinctly remember noticing a strange scar on my mother’s upper arm, near her shoulder. It looked like a ring of small indents surrounding a larger one. For some reason, it fascinated me—but like many childhood curiosities, I eventually forgot about it. Years later, while helping an elderly woman off a train, I noticed the exact same scar in the same spot on her arm. My curiosity returned instantly, but there was no time to ask her about it. Instead, I called my mother.

She laughed and reminded me she had explained it before: the scar came from the smallpox vaccine. Smallpox was once a devastating viral disease that caused fever, severe rashes, and often death. During the 20th century, it killed roughly three out of every ten people who caught it, according to the CDC.