A small mark on someone’s arm can carry a story that stretches across generations and borders. If you have noticed a similar round scar on relatives, friends, or people you meet while traveling, childhood vaccination may explain the resemblance.

Two vaccines commonly associated with lasting upper-arm scars are BCG, which protects against tuberculosis, and the traditional smallpox vaccine. Although their marks can look similar, the vaccines—and the reactions they cause—are different.

BCG contains live, weakened bacteria. After vaccination, a small sore or blister can develop at the injection site. As it heals, it forms a scab that may take weeks or months to disappear, sometimes leaving a small, flat scar. That visible reminder can remain long after someone has forgotten receiving the injection.

Traditional smallpox vaccination used a distinctive technique. A clinician repeatedly pricked a small area of skin with a two-pronged needle carrying the vaccine. A lesion then developed, progressed into a blister, and eventually scabbed over. When the scab fell away, a small scar remained.

The vaccine also differed in a key way: the traditional smallpox vaccine contained live vaccinia virus, a relative of the virus that causes smallpox. It did not contain smallpox virus and could not give someone smallpox. Describing it simply as a weakened version of smallpox would be inaccurate.

Where and when a person grew up helps explain which vaccination they may have received. Britain stopped recommending routine smallpox vaccination for children in July 1971, while the United States ended routine vaccination in 1972.

This helps explain why smallpox vaccination scars are more common in older generations. BCG, meanwhile, remains part of childhood vaccination programs in many places. The World Health Organization recommends a birth dose in countries or settings with a high incidence of tuberculosis.

Elsewhere, vaccination may be offered selectively to children at greater risk. These differences help explain why familiar-looking scars appear across countries and age groups.

For travelers, the mark offers a small glimpse into the public-health histories that people carry with them. Its story may begin with a childhood appointment, yet connect someone to vaccination programs that reached far beyond their hometown.