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.”

Partly because the vaccination process was so brutal, and partly because anti-vaccination crusaders exaggerated the risk of contracting tetanus or syphilis through the vaccine, there were plenty of people who tried to avoid vaccination by any means necessary. The most common tactic was to buy a fake vaccination certificate.

Even as late as 1904, an article in The New York Times headlined “Vaccination Certificate Frauds” reports on “an extensive traffic in worthless certificates of sufficient vaccination by east side physicians” perpetrating a “petty swindle on the poor, ignorant and credulous.”

With all public schools requiring proof of vaccination, anti-vaccination leagues circulated names of doctors who would sign a piece of paper saying that a child was medically “unfit” for vaccination. If parents didn’t want to pay the doctor, they forged the medical certificate themselves.

In the overcrowded tenement districts of cities like New York and Boston where smallpox spread with deadly speed, health officials enlisted policemen to help enforce vaccination orders, sometimes physically restraining uncooperative citizens. Frustrated with the widespread resistance to vaccination, these vaccine squads began to ignore certificates altogether and go right to the source.

“Because certificates could be so easily forged, they’d insist on seeing the vaccine scar,” says Willrich. “Vaccine scars readily served as a physical form of certification.”

In 1901, respected physician Dr. James Hyde of the Rush Medical College in Chicago wrote an editorial urging public health officials to do everything in their power to eradicate smallpox and proposed using the vaccination scar itself as the sole entry ticket or “passport” to civic life in America.

“Vaccination should be the seal on the passport of entrance to the public schools, to the voters’ booth, to the box of the juryman, and to every position of duty, privilege, profit or honor in the gift of either the State or the Nation,” wrote Hyde.

In schools, factories, and halls of government, as well as aboard immigrant ships arriving at U.S. ports of entry, those who couldn’t produce a “fresh” vaccine scar—signaling inoculation within the past five years—would be vaccinated on the spot.

In 1903, the state of Maine issued a decree that “no person be allowed to enter the employ of, or work in, a lumber camp who can not show a good vaccination scar.” In that same year, industrialist Henry Clay Frick ordered all employees at his Pittsburgh-area steelworks and their families to show a scar or be vaccinated.

“This order would have affected 300,000 people,” says Willrich. “That’s pretty significant coming from one enterprise.”

As late as 1921, when Kansas City suffered a smallpox outbreak, a local newspaper reported that “‘Show a scar’ has been officially adopted as the passwords to lodges and other meetings.”

Anti-vaccination sentiments never went away entirely, though, and some Americans even took to forging their vaccination scars. They did it by painfully exposing a patch of skin to nitric acid to produce the same nickel-sized scab and scar.