No. It caught from contact and regular shampoo and bathing does not prevent head lice. Do not share combs or wear other persons cap.
Contacts matter. This is a socially influenced dx, not a dirty one. Kids hanging scarves & headgear together in one place at school; getting together for rough & tumble wresling or other contact sports, lying down on sleep mats at day care etc. Are events that increase transfer. Kids without contact with a source will not develop this only human paracite. Poor hygenic practices can increase potential for transfer.
No difference. Head lice are parasites and will affect people whether their hair is clean ir dirty.