You may have noticed it on a parent, grandparent or older relative: a small, round scar high on the upper arm that looks different from an ordinary cut or scrape.

For many people, that mark is a lasting reminder of a vaccination received decades ago.

One possible explanation is the smallpox vaccine, which commonly left a permanent scar. But the scar alone isn’t enough to identify the vaccine with certainty. BCG vaccination against tuberculosis can also leave a noticeable scar, particularly among people born in countries where BCG has been routinely given to  children.

Smallpox was a highly contagious disease caused by the variola virus. It typically began with symptoms including high fever and severe fatigue before producing its characteristic rash. The lesions eventually filled with fluid and pus before crusting over, and survivors could be left with extensive scarring.

Vaccination became one of the most important tools for eliminating the disease.

The traditional smallpox vaccine contained vaccinia virus, a virus related to the one that causes smallpox but not the variola virus itself. The vaccine therefore could not give someone smallpox.