Yeah, it’s just about the same thing as seeing people in their hospital scrubs on the train or out to eat. You have to think, “What infectious disease have these people come in contact with as part of their work?”
With cloth grocery bags, you have to think, “What communicable disease is being retained on that thing?”
Not just disease. I met my first cockroach when I was in college. My roomies and I discovered they were hitching a ride into our apartment in our grocery bags. Our local grocery store, it turned out, had a YUGE cockroach problem.
Those were in paper bags, however. And I still prefer paper and/or plastic to reusable. I’m more concerned about the bugs I can’t see.