There is proper gym etiquette, common sense if you will.
If you enter the cardio section and there is a row of 7 empty elliptical machines, do not hop on the one right next to someone else.
If you stinketh, weareth deodorant.
I fell victim to both last night. Some woman obviously wanted a new friend, but I moved. I've been sick for a week and close proximity to another in a deep-breathing environment wouldn't be fortunate for said individual....and she was grunting. Within 5 minutes el stinko came and occupied the machine next to me. I succumbed to the smell and ended up covering my nose with my shirt until I could no longer breathe. How could you not know? But by this time, all the machines were full, so I had nowhere to run (ha ha - pun alert!).
I would love for someone to invent a stink-o-meter you have to pass through, like security, in order to enter the gym. Sure, after one too many bean burrito's or one too few swipes of deodorant, all of us would fail at one point or another, but it would save a LOT of trauma!