Of all the germs kids are exposed to on the playground, there’s one they freak out about more than any other: cooties.

The word first appeared during World War I as soldiers’ slang for the painful body lice that infested the trenches. It went mainstream in 1919 when a Chicago company incorporated the pest into the Cootie Game, in which a player maneuvered colored “cootie” capsules across a painted battlefield into a cage. The cooties concept has been evolving ever since.

The most familiar incarnation has features of a real infectious disease even as it says a good deal about what 6-year-olds think of the opposite sex. Every little girl knows that boys have cooties, and vice versa. One catches cooties by—eww!—touching. Shrieking games of cooties tag transmit the contagion rapidly. It can be treated with an origami “cootie catcher,” but it is better to be vaccinated.

This requires a friend and a retractable pen. Your friend clicks the pen onto your arm while chanting “circle, circle, dot, dot, now you have your cootie shot.” Folklore archives and internet forum threads show that regional variations of the therapeutic regimen have emerged. In Louisville, the charm is “line, line, dot, dot, operation cootie shot”; in Los Angeles, kids “pinch, pinch” in lieu of the “dot, dot”; in Hawaii, the process is known as an “ uku shot.”

To historians and social scientists, the cooties phenomenon isn’t just child’s play. Kids, after all, are their own “semiliterate society” with their own cultural touchstones, says Simon Bronner, a folklorist at Penn State Harrisburg who has studied children’s traditions. The purpose of something like the cootie shot, passed down from generation to generation, “must be profoundly important if all these kids are choosing to participate in it,” says Tok Thompson, an anthropologist at the University of Southern California who studies modern folklore.

Play helps kids make sense of new ideas, experiences and emotions, not to mention traditional gender roles. The cootie shot itself is part teaching tool, part coping mechanism. Bronner has observed the emergence of this form of cooties in the 1950s, when the polio vaccine became ubiquitous, and a spike in its popularity in the 1980s, during the height of the AIDS epidemic.

Nowadays, cooties also reflect other concerns, particularly physical appearance; an obese child, for instance, might be said to have cooties. There’s a greater emphasis on body shaming, Bronner says. Like a real virus, cooties mutate, and they’ll likely be around for as long as children have insecurities to play out.

When Women Had Cooties

Cooties weren’t just for kids. As early as 1921, a dice game called Cootie was a favorite at wedding showers. Women competed in teams of two to draw the very bug that had tormented many a husband-to-be during the war. One woman threw a die; the other was the artist. A six earned the team a cootie body; a five, the head; a four, one of six legs; and so on. The game was still popular among brides in 1949 when a Minnesota inventor created a 3-D version (below) in which players built cooties from colorful plastic pieces. It became a big hit with children and is still in production today.