Yeah. It's like anti-biotics. There's the proof of evolution right there. Natural selection. The bugs (both kinds) that can survive the medication go on to reproduce more that likely will survive it and eventually we have bugs that can't be killed off by usual means. And with lice, even if you do everything you can, even if you are treating, gelling, combing, daily washing with dandruff shampoo (yes, that helps), and manage to pretty much eradicate them from your household, if everyone else isn't doing the same then it can all be for naught. If I get every lice and nit off of my kids' hair and they go back to school and play with their friends again, at least one of them probably having lice too (that they may have gotten it from in the first place and may not be getting treated as thoroughly), then the whole nightmare can start all over again.