"soap will kill them"

You'd ... lie to them? Good idea: "soap will kill them" would be more instrumentally useful than some actually true but more complicated explanation, and once they figure out you weren't truthful (at which point they'd probably have figured out other ways to defend against microbes besides the hygiene hypothesis) you would've taught them another implicit lesson: Don't trust the authorities!

So, two for the price of one, with the latter unlocking after the first lost its usefulness. Smart!