Lice Alert

Have you had a wave of head lice going through your child's school, camp or day care? Frustrating and tedious daily combing, nurse calls, parent whisper, days off from school, infinite laundry, empty shampoo shelves at the pharmacy, angry haircuts - all these efforts devoted to fighting a ferocious tiny insect.
How quickly do you think head lice could spread around if we do nothing?
Let's pretend that each day lice spreads from one child to three more. And that lice is smart and populates only "virgin" territories avoiding already infested heads. Say, there are 400 kids at school. And it all starts with a single nit in one child's hair.
How many days would it take for everyone at school to get infested?

P.S. No, we have not had any lice in our neighborhood in the past two years, but swine flu panic was a reminder.