Grandma Margie!

I tought so too, but i guess because its an infection movie, they just show different ways to infect people like the sneeze in the club. Like the handshake from the policewoman etc.

Pretty sure she got it the old fashioned way. When Harper asks her about her cold sore, her response was something like "wouldn't you like to know." She probably got the disease from her boyfriend, who doesn't appear in the movie.

I was under the impression that she got it because of the toothbrush, too. They made it a point to show that Riley's toothbrush touched his grandma's when he put it back. I can't remember if Riley had coughed up blood yet by that point, but it seems to me all it would take would be for him to have an open sore in his mouth, or even just trace amounts of saliva left on his toothbrush, to cause his grandma's infection since his toothbrush did touch hers. Same could be said for sharing that drink, but I leaned more towards believing it came from the toothbrush since they panned in on them touching. I know it's not the same thing, but my cousin's husband has Hep C - his doctor said they can share drinks all day long, but have to be VERY cautious to keep their toothbrushes away from each other, just in case of any cuts or abrasions in his mouth that he's unaware of, which could pass the infection on to my cousin. So maybe this could apply to the movie as well, I don't know. Again, I know it's not the same thing, but that's one of the first things I thought of when I saw their brushes touch, and figured that's how Granny became a Grand-Zombie. :)