Many people have a small, round scar on their upper arm, a lasting mark from the smallpox vaccine, which was commonly given before the 1970s. This vaccine used the live Vaccinia virus to create immunity against the deadly Variola virus, the cause of smallpox.

After receiving the shot, “blisters appear at the injection site, which eventually heal and leave a circular scar,” explains the original article. The vaccination process involved multiple needle pricks that caused blisters, leading to a brief swelling, then a lump resembling a mosquito bite. This lump grew, oozed fluid, and eventually healed into a scar that remains.

Smallpox was eradicated in the Western world by the early 1970s, and vaccinations stopped in the 1980s due to no further risk of exposure. The scar serves as a historical reminder of a once-deadly disease.