Theirs always seem a bit of confusion of how people contract head lice. A few ways of getting head lice is through head to head and hair to hair contact; examples are selfies ( heads are together), hugging, cuddling etc. Basically if you are in close contact with others. People are most likely to get head lice from people who they know and are comfortable with ( such as a friend, relative, classmate etc) rather then a stranger.
Like I mentioned in the first collection ( what is head lice?) head lice do not jump or fly they just crawl really fast. So if two people are taking a selfie and the self takes about 8 seconds to take Lice can take about 3 seconds to crawl from one head to another. So they are pretty quick.
Children ( especially preschool and elementary age ) are the mostly people to get head lice because they do not have a concept of personal space.
But with the popularity of selfies I have seen a lot of teenage girls with head lice as well.