This is a bit long but might save some people the hassle we went through - after an infestation we repeatedly failed to clear I did some basic research, and treating according to that worked first time.
What I found was:
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Lice can start laying eggs as quicky as 7 days after hatching - NOT 14 or more days as many websites say. Look at (eg) UK dept of health website for the 7 day number.
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It seems to be very hard to guarantee killing/clearing all unhatched eggs by treatment/combing. All evidence I found is that NO treatment can be relied on to do this.
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But there are treatments that do seem to clear all LIVE lice. I personally have found Hedrin Once works well for this.
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Lice eggs take 7-10 days to hatch.
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Although headlice are not supposed to be able to survive for long when not on a human head (they dehydrate), they can last for 24 hours at least if the climate is reasonably moist.
And the consequence of that is:
A) Treating once, or twice 1 week apart won't work since you will likely still have unhatched eggs and get back to square one.
B) But if you treat at days 1, 7 and 13 then no headlice live long enough to lay eggs, and all pre-existing eggs have hatched. So this should clear everything - and this did indeed work for us.
C) You should change bedding, clothes and towels on treatment days. I know it's a pain but there does seem to be a chance headlice can hang around for a day without being on someone's head.
The trick is to not leave more than 6 days between treatments, and for the full span of treatments to cover the 10 day egg hatching period. So days 1, 6, 11 should work too for instance.
You will still need to comb dead lice and eggs out of the hair. The Nitty Gritty comb seems to work well for us. But the detailed combing is clearing up afterwards, rather than a necessary part of the killing process. It is quite easy to comb nits/dead eggs out while Hedrin Once is applied (if that's what you use), so worth doing as much as your child can stand at that point.