Diseases that the opposite sex has that gives you a reason to not be around them. When you get older, cooties becomes known as "STDs."

US slang that references childhood:

Cooties are an imaginary affliction from childhood.

When children reach the age where they notice the sexes are different the children claim a member of the opposite opposite sex will give you "cooties" if they touch you.

Its a way of little girls telling other little girls not to play with boys and vice versa

Highly Contagious, Viral batecteria that breed inside young females and are released into the air via sweat glands when in the proximity of other young boys.
Various forms of protection include, Sprays, Suits, and name calling.

Imaginary infestations of the truly un-cool.

an invisible bug thats on boys and girls
you get cootiefied after hangin out and being close to the opposite sex
you get very itchy

According to adults (generally parents of young children): small, hairy monkey that lives in the back of one's throat; also known as "throat troll". Commonly transmitted by young children expressing affection (i.e. kissing a.k.a. swappin' spit, tonsil hockey, etc.) to the opposite sex (or "crush").

Slang for pubic lice at the turn of the century.