microbes

Today my daughter asked me about the microbes. Children have been scared at the kindergarten with saying that those who does not wash hands have thousands and even millions microbes on hands.
Well, in fact they could have explained what the microbes are.
And how to explain that some of them are not harmful for us but instead useful.
Those ones, for instance, inhabiting our bowels, but how did they get there if not from the dirty hands?
Well, I am lucky the school is only in some years.