At the turn of the 20th century, the United States was in the grip of a full-blown smallpox epidemic. During the five-year outbreak from 1899 to 1904, government health officials confirmed 164,283 cases of smallpox, but the real numbers may have been five times as high.

To slow the spread of the highly infectious and often deadly virus, there was a nationwide push for smallpox vaccination. In cities and states with the worst outbreaks, vaccination was compulsory and official certificates of vaccination were required to go to work, attend public school, ride trains or even go to the theater.

The mandatory vaccination orders angered many Americans who formed anti-vaccination leagues to defend their personal liberties. In an attempt to dodge public health officials, who went door-to-door (often with a police escort) to enforce vaccination laws, some anti-vaccination activists would forge certificates of vaccination. Unable to tell if certificates were legitimate, health officials fell back on physical evidence: they demanded to see a vaccination scar.

Following a technique first developed by Edward Jenner in the late 18th century, smallpox vaccination in 1900 meant scoring the skin of the upper arm with a lancet or knife, and then dabbing the wound with live virus. Vaccine makers in 1900 still sourced their virus from oozing cowpox sores on the underside of calves.

“The vaccine recipient would start to feel quite sick, usually with a fever and a very sore arm,” says Michael Willrich, a history professor at Brandeis University and author of Pox: An American History. “The vaccine site would become more and more irritated, a scab would form, fall off, and what was left behind was a small scar roughly the size of a nickel. And that’s how you’d know that the vaccination took.”