How do you feel about strangers touching your baby?

I was in the grocery store today carrying my two month old son in a front carrier facing me. A middle age woman came up and just tried to touch him. I turned my shoulder to her blocking her, pretty much as a reflex. She hadn't even said a word to me. She rolled her eyes and gave me a smug bemused look like I was being silly and over protective. I don't know her. I don't know if her pertussis vaccination is updated and honestly, I just don't like strangers touching my baby. What do you think?