Good question, room. The way it was told to me, is that it started with rats, way back when. All I know is, they were prevalent in my mother's day, and when we grew up, they were not. As I said, we were all warned about putting anything that was not ours, on, or around our heads. Getting lice at school was unheard of.
When I got older, more than once I'd be at a club, concert, or other event, and while brushing my hair in the bathroom, some idiot would ask to use my hairbrush. Usually my response was "how do you know I don't have lice, and my hairbrush would give them to you"? Or I would say "that's not sanitary, so I don't think so." Usually they would then ask someone else. Eventually someone would actually let them use their hairbrush.
When my daughters went to school, I was actually shocked to get the warning letter that they were going around. I wondered why people don't teach their kids things anymore that were commonplace when I was growing up. Especially that boys are to lift the toilet seat, then put it down when done, and that girls are not to sit on toilet seats. You'd be surprised how many parents have never taught these things to their children.