If they get them, you use a special shamppo (at any pharmacy), and then you have to comb our all the nits (or eggs) or the whole thing just starts over again. PITA if you have a littel girl with long thick hair.
Wash all the bedding, bag up stuffed animals,etc in palstic bags for a couple weeks (or wash the eashable ones), vaccum thoroughly.
Animals don't get them for some reason.
If there is a known outbreak around, put girls' hair up in a pony or bun, and use hair gels and sprays liberally. It repels the little buggars. Also, drill into your kids' head to NOT share caps/ brushes or anything.