'Wow it must be so amazing to be soooooo smart and sooooo clever that without a medical degree or any training you can look at my baby who you have never met before, have not even examined and know EXACTLY what is wrong with him. I wish I was as clever as you!'
We all know that such a clever comment would not come out of our mouths at the moment of confrontation but gee it would be cool to have said it.
Hope you got your shopping done and your LO is feeling ok!
I wouldn't bother with that. If you ever saw one of these rashes it looks exactly like measles. Few things look like it. The random person will be freaked out because they will think it is measles. You won't be doing them any harm though.
On a side note, you may even do some good - scare some people into getting their vaccinations.