... then they're probably the source.
Surely the main difference between kids (at least at primary school) is not whether they get them, but whether it's recognised and treated? Are nits really that picky about whose head they infest? If 40% of kids at any one time have threadworms, then is it really possible that so many kids are thought by their parents to never get them?
My kids seem to crawl with parasites regularly. I treat it thoroughly and then, a few months later, they're back. I'm not alone in this, am I?