Why don't African-American/Canadian children get head lice?
It's true. It is extremely uncommon for African-American children to get head lice. What makes this strange is that Black African children get head lice. What's the difference?
This 2001 article from the University of California explains the reason behind this. North American head lice prefer hair that is round in cross-section - like the hair of Caucasian and Asians. But the hair of African-American's is ovoid (oval or egg-shaped) in cross-section. So they don't like it that much.
The hair of the Black African is also ovoid. But the head lice in Africa have adapted to this shape of hair. Because this is what head lice do. They adapt. In other words, given some time, I'm sure North American head lice will be equal opportunitiy parasites.