Cooties is a term in North American English used by children for a real "germ" said to infect through contact. The term may have originated with references to lice, fleas and other pests. A child is said to "catch" cooties through any form of bodily contact, proximity, or touching of an "infected" person or from a person of the opposite sex of the same age. The phrase is most commonly used by children aged 4–10; however it is also used by many others older than 10 years of age.
The word may be derived from the Malay word kutu for a biting insect. The word kutu for louse appears in a Malayan–English dictionary as early as 1812. Related languages use kuto, kuta, kutta, and such variations for louse, including kuti in British New Guinea. While kutu was the form used in the aboriginal language in New Zealand, they now use kuti as an alternative spelling for the English slang cootie.
The earliest known recorded uses of cooties date back to the First World War, including a 1917 service dictionary. Albert Depew's World War I memoir, Gunner Depew (1918), includes: "Of course you know what the word "cooties" means ... When you get near the trenches you get a course in the natural history of bugs, lice, rats and every kind of pest that had ever been invented." Similarly, Lieut. Pat O'Brien's 1918 memoir Outwitting the Hun: My Escape from a German Prison Camp refers to "cooties," meaning body lice, which in his case had been caught in the prison camp in Courtrai. Lice were of course rife in the trenches on both sides of the conflict, and highly contagious.
From its original meaning of head or body lice, the term seems to have evolved into a purely imaginary stand-in for anything contagious and repulsive.
The lice of the First World War trenches nicknamed "cooties" were also known as "arithmetic bugs," because, "they added to our troubles, subtracted from our pleasures, divided our attention, and multiplied like hell."
For ages 5 onwards, Cooties are known in Denmark as "fnat," or "pigelus" (literally "girl lice") and "drengelus" ("boy lice"), and in Norway "jentelus" ("girl lice") and "guttelus" ("boy lice"). In Sweden and Finland it usually refers to girls, where they are known as tjejbaciller" (literally "girl bacillus") and "tyttöbakteeri" ("girl bacteria") respectively. In Britain, the term "lurgy" (or lurgi or lurgie) is used by children in a similar way. This is why, in the Harry Potter book, Luna Lovegood refers to the Slytherin team as suffering from "Loser's Lurgy".
Cooties is probably the term which the Malays refer to as "kudis". The term "kudis" is widely used by all ages and even used as a medical term to describe the real skin disease such as scabies (kudis buta) and impetigo.
Children sometimes "immunize" each other from cooties by administering a "cootie shot". One child typically administers the "shot" by reciting the rhyme "circle, circle / dot, dot / now you've got the cootie shot" while using an index finger to trace the circles and dots on another child's forearm.
In some variations,a child may continue to then say "circle, circle / square, square / now you have it everywhere", in which case the child receives an immunization throughout his or her body. These variations may continue to a final shot where the child then says "circle, circle / knife, knife / now you've got it all your life", "circle, circle / dot, dot / now you've got the cootie shot" or "circle, circle / fire, fire / now your shot will never expire", or "nickel, nickel / dime, dime / now you've got it all the time" while using their index finger to draw vertical lines on the other child's forearm.
Alternatively, cooties can be immunized through one child creating a square using his or her index and middle fingers (making a peace sign in each hand and laying one on top of the other). The other child then pokes his index finger through the square, at which point he becomes immunized from cooties infection.
In playground lore, the power of a "cootie shot" is not limited to use as an immunization. The "victim" of cooties may receive a cootie shot as treatment, at which time the cootie shot may "cure" the disease. In this way, the cootie shot acts more like an antidote rather than a vaccine. When used as an antidote, sometimes a "cooties shot" is actually just a punch to the upper arm which then "cures" the punched one from the "disease".