You know, with chickenpox, I'd be annoyed at the dismissive thing, because it isn't *all* about whether the illness can turn serious. It's also about how annoying it is while you have it! Kids always wanting to scratch at spots, medicine to put on it, kids scratching themselves by trying to itch, kids who can't sleep because they itch so much, so it means *you* can't sleep... it's not all about whether it's going to be that 1/1000th chance or whatever that it turns serious and leads to brain damage, it's also just not fun, even when it's "just" chickenpox. So I'd be a little annoyed with it on that aspect.
However, I do think the fact that you sound just a bit dismissive of it yourself, *and* that they work with the really serious cases so they're probably thinking, "Whew, it's *just* a regular illness and he's obviously not a serious case or she'd say so, and given all the really terrible cases we have, she should be relieved and not fretting about it," or something like that. I do think it's odd that they ask if they really don't care, though.