I love the explanation by @pled. In addition, I say the same if asked why I am wearing a mask. I might add (and sometimes do) "If I get even a common cold, I can end up with a serious infection, and I just won't take the risk."
By the way my daughter is on immune suppressing meds, wears a mask in most indoor situations - including at work, and she always graciously explains to people - especially kids.