Age 15: "Ewww... She has herpes, I can't sleep with her!"
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
Various forms of protection include, Sprays, Suits, and name calling.
In short, the other sex has them, you can get them, and you do not want them.
Boys: Eww, Josh has cooties!