Millions of people around the world have noticed a small, round scar on their upper arm that looks remarkably similar from person to person. For some, it has been there for so long that they barely notice it anymore. Others remember seeing the same mark on their parents, grandparents, classmates, or neighbors and wondering why so many unrelated people appear to share it.

The explanation is surprisingly simple: for many people, this is a vaccination scar. One well-known cause is the BCG vaccine against tuberculosis, which has historically been administered routinely in many countries. Unlike most modern injections that leave no permanent visible mark, BCG is given into the skin and commonly produces a small local reaction during healing.

After vaccination, a small bump can develop at the injection site. It may later form a tiny sore before gradually healing over several weeks. Once completely healed, some people are left with a distinctive round or slightly irregular scar. Its size and appearance vary, but the characteristic mark can remain visible for decades.

Depending on a person’s age and where they grew up, a similar upper-arm scar can also come from older smallpox vaccination programs. Smallpox vaccination frequently produced a noticeable permanent scar as well. Because routine smallpox vaccination ended after the disease was eradicated, these scars are now especially associated with older generations in countries where the vaccine was once widely administered.

So there isn’t a mysterious condition causing millions of people to develop the same mark. Those small circular scars are usually reminders of past vaccination—most commonly BCG in many parts of the world, or smallpox vaccination among older generations. A tiny scar that seems insignificant today can actually reveal a fascinating piece of public-health history.