Answer: Lice

If you spent even a wee bit of your childhood in the U.S., then you most certainly encountered the dreaded, but entirely imaginary, childhood disease “cooties”. While the rules of transmission and infection vary quite wildly between groups of children and variations of the “disease” with differing names are found all over the world, there’s one thing very concrete about cooties: the way the term entered the English language.

The word first appeared in English when used by British soldiers in World War I to refer to the lice that ran rampant in battlefield trenches. Although the origin of the slang term isn’t precisely known, it is believed to have come into the soldiers’ lingo by way of the Philippine, Malaysian-Indonesian, and Maori word kuto or kutu, which refers to a parasitic biting insect.

The term cooties in turn jumped across the Atlantic, again by way of military activity, when American soldiers returning from South Pacific service alongside British soldiers popularized the term in the U.S. during the 1950s.