Myth: You get nits only if your hair is dirty.
May 23rd 2017 by Tony Maleedy.
Not true. A person with perfectly clean hair can pick up head lice (nits are the eggs of head lice) as easily as a person who has not washed their hair for a while. Washing the hair with normal shampoo will tend to remove some of the lice, but not all, and certainly will not remove the nits which the lice have firmly cemented to the hair shaft. You need a special head lice treatment shampoo or lotion to completely clear this problem.
The science bit:
Head lice are not there because they like dirt but because they like blood – and we all have that. The head louse feeds exclusively on human blood and once removed from the scalp and its supply of food, and warmth of the skin, soon dies.