my daughter used to be continuously infested for a many month (together with a sore throat and runny nose)around the age of 10, we got rid of them, next week they were back. strangely enough they would never leach onto me.
i figured that it had to be something internal that made her so attractive to headlice, so i took her off all dairy products and presto.... all gone and never to be seen again.
my guess is that the dairy altered the ph of the scalp and made it hospitable to the nasty little creatures.
just some food for thought