If you are unwanted by someone who you wish to know or make friendship with (girls mostly), they say you got cooties.
is humiliated by friends of the girl when he gets curious to know what presents the girl got. On among the girl's friends says "He's got Cooties".
Also some childish urban legend that a person contracts lice from contact with females. Probably dates back to a time when people never bathed and the risk of getting a disease was high.