You know you have just tempted fate, right?!
I have two school-aged DC and TOUCH WOOD we haven't had them - yet. We spent a couple of weeks over Christmas / summer with my best friend and her family. Her DD, same age as my eldest, has recurring nits.
The kids were sharing a bedroom, and running in out of each other's tents when we were camping, and somehow we miraculously escaped. DD has long hair, so I kept it tied up in a tight bun for the duration.
DD always has her hair pulled back at school and on play dates. I'm guessing this makes a small difference, but can't actually be a game changer - she has thick wavy hair that even in a pony tail must still be susceptible.
No threadworms - yet - either.
I don't see how it can be avoided forever, these things are a part of childhood, aren't they?
And I don't think you're missing the signs though - I think that's pretty much impossible!