Ew! Girls! Cooties!
A cootie is actually a louse. The plural of cootie is cooties. The plural of louse is lice. Some people, male or female, have lice, but I certainly hope none in my immediate vicinity do.
They are asymptomatic carriers of stupid disease, which afflicts guys around them. Symptoms include: Drooling, staring, NOT GETTING ANYTHING DONE, and hitting on the female in question. Happy, PetersSmith? I fixed it.