When others have head lice.
So your cousin has head lice. Should you let her come for a visit? You find out that 3 kids in your child's classroom are totally infested. Should you keep your child home or should you raise a stink with the principal? The mother of your son's best friend just called - her kid has head lice. Do you keep the boys apart?
When someone with lice is coming for a visit, don't isolate yourself - just take some simple precautions. If you or your children have long hair, pull the hair up in a ponytail. If the visitor has long hair, ask him/her to pull it up (this is easier to do when your guests are children). Address the issue with your guest at the start of the visit. If your guest is staying over, give him/her fresh bedding and wash and dry the bedding in high heat each day. Put towels over the tops or arms of the couches - anywhere where someone might lay their head. Put your brushes and combs away. Make sure no one shares hats or hair accessories. Set some ground rules about contact - no hugging when possible (But hey, if my friend was really upset, she might need a hug, and I'd give her one, head lice or no head lice.) And then do regular head lice checks on yourself and members of your household.
With regards to the lice in your child's classroom, talk to the other parents, or talk to the teacher to make sure that everyone is aware of the problem. Offer information and support. And by all means, give them my number!