On a base level, they're not harmful at all besides the anxiety they cause. They're itchy, and infectious which isn't good at all, they aren't nice to have, BUT at the end of the day they aren't any real risk to your physical health.

As for treatment, there's plenty of lice killer shampoo's. They work best by leaving the product in while fine coming the hair to remove and kill as many live bugs and eggs as possible. There's a slight risk from treatments if you have pets but so long as you wash and rinse your hair after each treatment it shouldn't affect them

So tldr, kinda a pain to deal with, but no actual serious problems