The events that led teams of helicopter-borne vets to pelt the Swiss countryside with vaccine-impregnated chicken heads began in 1939. Two things were then sweeping through Poland: the Nazis, and an epidemic of rabies carried by red foxes. Every year, the wavefront of disease advanced southward and westward by several dozen kilometers, hitting country after country. In March of 1967, it reached Switzerland.

The epidemic was a huge problem. Rabies is caused by a virus that spreads through the bites of animals and targets the brain. Unless infected people get a (really expensive) vaccine right away, the disease is almost always fatal. So, something had to be done about the foxes. The usual methods—poisoning, trapping, gassing, and shooting—weren’t working. The alternative was to vaccinate them.

Vaccinating wild animals against disease was a new concept, and American scientists had been the first to try. Initially, they tried capturing animals and immunizing them by hand, but that proved too laborious and costly. So in the 1960s, scientists started looking for ways of persuading animals to vaccinate themselves. George Baer from the CDC began by adapting the Coyote Getter—a buried gun-like contraption that would shoot cyanide into the mouths of inquisitive coyotes. Baer tried to replace the cyanide with vaccines. It didn’t work.

William Winkler developed a more effective but equally wince-inducing option: the Vac-Trap. Picture a foot pad, connected to a spring-loaded armature that held a vaccine-filled syringe. Step on the pad and—bam!—syringe in the leg. It worked, but as Winkler wrote, “it proved too hazardous.” Anyone, including unlucky hikers, could trigger the giant needle. (Winkler once scattered 130 Vac-Traps on a deserted beach only to be told that the U.S. Navy would be using it for a mock-invasion training exercise; he offered to remove them but officials argued that “the hazards would serve as an additional measure of the invaders’ prowess.”)

Abandoning this route, researchers started trying to create oral forms of the vaccine that could be packaged into edible bait. After years of development, the first successful laboratory trials were done in 1971, using a live but weakened rabies virus belonging to the SAD strain. Then, it was just a matter of finding the right bait. They tried everything from dog biscuits to sausages to eggs, but European scientists eventually settled on disembodied chicken heads, with vaccine capsules hidden under their skin. What better attractant for a fox?

The would-be vaccinators had to deal with misgivings among both colleagues and the public. They were concerned that the live viruses in the vaccine would continue to spread on their own, eventually evolving into new virulent forms. To assuage these fears, Franz Steck, a charismatic veterinarian at the University of Bern, left several of the vaccine-infused chicken heads on a small, rabies-free island. None of the local rodents became infected. There were no unexpected outbreaks.

In 17 October, 1978, Steck deployed the baits in a real field trial—the first of its kind. At the time, the rabies epidemic was spreading along the east shore of Lake Geneva, so Steck’s team created a grisly firebreak of 4,050 chicken heads.

The heads also contained a chemical marker—tetracycline—that could later be found in the teeth and bones of foxes that were shot by hunters. When it became clear that the foxes were actually taking the bait, the initiative garnered more interest, money, and effort. The team dispersed more baited heads, mostly by flinging them onto roadsides and paths. For more remote areas, they used helicopters. From 1979 to 1984, chicken heads would rain down on the countryside.

The program was a success. Over four years, the team dispersed some 52,000 baits, and wherever the heads landed, rabies disappeared. And so, unfortunately, did Steck—he was killed in 1982 when his helicopter crashed while delivering baits.

In 1983, Germany started their own vaccination efforts. By the mid ’90s, 16 European countries were taking part. New and more effective vaccines were developed. Aircraft became more commonly used; France abandoned ground units and relied only on helicopters as of 1988. And while the Swiss were baiting some 150,000 chicken heads a year, their cottage-industry approach was impractical for bigger countries like Germany; with characteristic efficiency, they replaced the birds with mass-produced tablets of fish or fat. They churned out millions every year. (Switzerland stuck with chicken heads until 1991, before switching over.)

These methods worked. Typically, the campaigns slashed the numbers of rabid foxes by 90 percent within a decade, and sometimes in just half that time. In 1983, Europe reported 23,000 cases of rabies in animals; in 1995, they were just over 8,000.

Switzerland, the country where it all began, saw a brief resurgence in the 1990s due to booming fox numbers, but they brought it to heel by doubling the density of baits and by specifically targeting fox dens to vaccinate newborn cubs. By 1996, they were rabies-free. By that time, they had scattered 2.8 million baits over their country, and Europe as a whole had dispersed around 74 million. Rabies had been outfoxed.