If you have pets, they can be "carriers". They can't be infested but the lice can live on them for a couple days which is the same for carpets, rugs, furniture, mattresses and stuffed toys.
Treat your pets with a commercial product as well as carpet, mattresses and furniture. Put ALL clothes and stuffed toys in garbage bags and tie a big knot to seal it shut. Make sure you take clothes DIRECTLY to the HOT water washer with no "left-overs" on the floor for the next load. Most stuffed toys can not be washed but they can be put in the dryer. Use a laundromats big dryer, as they can get hotter then ones we use at home, and let them go for 20 or 30 mins in there.
Mayonnaise works just as well as the commercial products on everyones hair. You must do EVERYONE in the house. Cover the hair/head thick and good with mayo, place plastic wrap on top to seal and nuke a bath towel for a few seconds so it's nice and warm as you wrap the haed. Leave on for at least 30 mins and up to an hour. Rinse with as warm of water as everyone will let you. Use a fine tooth comb and check the whole head. Wash with a good cleansing shampoo, Suave has some good ones. Now for the important part! Rinse with apple cider vinegar. Yes, V-I-N-E-G-A-R! Continue to rinse hair with vinegar as long as you wish. Lice HATE vinegar and will avoid it at all costs! Vinegar also helps to keep your hair a natural color when using a chlorine pool in the summer, gives hair a soft feel and a silky shine.
I am a former cosmotologist as well as a daycare provider. Once when I wasn't sure where lice had come from, I made a rule that ANYONE entering my house would be checked BEFORE they came in. Some parents didn't like this but as I said to them "at least you will know it's not your kid". No one argued after that. LOL!
During the "season", I tend to check kids hair as they walk in still to this day! LOL! Teenagers think I'm a bit unhinged but I haven't had lice in my house in over 10 yrs!