I go to the gym because it keeps me healthy. It strengthens my muscles, my heart, and my immune system. So I don't appreciate it when flu-infested Francine shows up and starts huffing and puffing on the treadmill next to me. I know that some people believe that exercising can help get rid of a cold faster, but there's a time and a place.
There usually isn't a whole lot of ventilation in the gym, and I don't need to be breathing in the germs you're trying to get rid of. I also don't want to touch anything you touched, and no — wiping the equipment down with your sweaty towel doesn't quite cut it. I especially don't appreciate you stuffing your used tissues in the drink holder (I seriously saw this the other day).
So do everyone a favor — if you're sick, stay away from the gym. If you're up for exercising, do a fitness DVD, some weight lifting at home, or go for a bike ride or jog outside. Then you can come back to the gym when you're germ-free.