Sorry, I can't think of much worse than some snot ridden flu infected person pulling out a rag full of old snot standing next to me in a coffee shop.
They roll it up put it in their pocket and wait a few seconds until next time their horrid fluids threaten to leak on the floor.
One, I would rather they were at home, but, if they insist on trying to infect the world at least blow their nose on a few tissues and dump them in a bin.
At least with the tissues they aren't going in the wash with the shirt that is brushing up against you in the service queue.