A very large percentage of infections are viral and antibiotics would be of no use. Hence the reason antibiotic abuse in humans is so common.
I suspect antibiotics in livestock/food supply are an even greater contributor to resistance, because they are used as preventative medicine in that sector, but really, neither is good.
So are his grandparents going to catch a cold from the trees? If they don't have contact due to blocked roads then I think they can rule out viral infections?
Can't rule it out. You don't have to be showing symptoms to be a carrier, and you can be a carrier for extended periods. It's quite possible (in fact likely) that there are plenty of virus sources waiting to get you, including inside yourself.
I suspect antibiotics in livestock/food supply are an even greater contributor to resistance, because they are used as preventative medicine in that sector, but really, neither is good.