MRSA, chicken pox, strep throat, pink eye, the flu – children are pretty much a walking Petri dish of potential pathogens. Yet ironically enough, the one dreaded bug every child fears the most is the one that doesn’t actually exist: cooties. Have you ever wondered how this cooties thing got started in the first place?

The first reference to “cooties” emerged during the first World War, when it was used to describe a painful body lice that afflicted soldiers in the trenches. In 1919, the term gained popularity when a Chicago company came out with the ‘Cootie Game,’ a little handheld gadget that tasked players to “capture the cooties” by moving little balls into a cage. This led to other types of cootie products, including one that’s still in production today: a set that allows children to build colorful cootie bugs from an assortment of plastic pieces.

Cooties soon became a common feature of tag, especially those games played between boys and girls. If a child got tagged they would need a friend to give them a cooties vaccination. A playmate would pretend to give them a shot using a retractable pen while chanting, “Circle, circle, dot, dot, now you have your cootie shot.”

Ideas regarding the transmission of cooties have changed over the years. Cross-gender touching–girl to boy, boy to girl–is a common theme, reflecting children’s sexual anxieties. This later expanded to include certain “undesirables” as well: overweight or obese children, for example, are commonly labeled as harbingers of cooties, as are those who have an odd physical appearance or other characteristics that set them apart from peers. For many unfortunate children, the term cooties is weaponized, and still brings up painful feelings to this day.

The cooties phenomenon also tends to enjoy a resurgence whenever there are concerns about transmissible illness, such as in the 1950s when the polio vaccine first became available, or later during the height of the AIDS epidemic. Since kids tend to explore real-world anxieties through play, cooties are the perfect way for kids to examine tensions about touching and transmission.

So there you have it: the origin of the world’s most infamous imaginary illness. When you go to sleep tonight, be sure not to let the cootie bugs bite!