As a child, I vividly remember noticing a peculiar scar on my mother’s upper arm, near her shoulder. It was a ring of small indentations surrounding a larger mark. 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 immediately resurfaced, but there was no time to ask her about it. Instead, I called my mother.

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

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 received the vaccine—and it left a permanent mark. That scar became a visible sign that someone had been protected against smallpox.....