Why it's a mistake: If you're getting your sweat on in a public place like a gym or yoga studio, where you share equipment or touch things commonly touched by others such as doorknobs or bathroom faucets, it's easy to pick up germs on your hands. If you touch your face during or after your workout, you might as well buy a box of tissues and some chicken noodle soup and plop your sick self on the couch.
What you should do: Be very conscious about not touching your face. Disinfect equipment when possible, and use a towel you brought from home to wipe off any perspiration. As soon as you're finished working out, head to the bathroom to wash off any germs from your hands. If you can't get to a bathroom, use antibacterial wipes or gel.