Among the most common head lice causes are lack of personal hygiene and contact with a person who has lice. You are never safe with the last reason, as you cannot know for sure if this or that person has lice, unless it is very obvious. But in modern world it is hard to imagine someone who is so neglectful to his or her health that will expose his or her hair full of lice and nits to the public without doing anything about this delicate problem.
As for personal hygiene, this cause is a bit exaggerated. It is not enough to have shower and wash your hair regularly otherwise people would forget about lice in civilized countries forever. Water neither kills lice nor washes away their eggs. So if by any chance you got a head louse, it can easily settle on your hair and survive your daily water procedures. Well then, how do people get lice? The answer is clear – from other people, who already have lice. It is also possible to get lice from infected objects, but without a human they live not more than for two days.