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People who cough and sneeze freely
The guy next to me at work is full of a cold, just sitting there, openly spluttering his guts all over his desk.
Not too fussed about how disgusting he is, but in terms of contamination fears, I'm fair worried. It seems that I'm one of the only people without a cold in the office and was hoping to get through New Year's Eve without one. Anything after then, no problem.
Think I'm just gonna say "Cover your mouth, you filthy get" next time which will hopefully be a half-joke half-serious hint. What can you do if that fails?