If you really think about what goes on in a gym, it's far less sexy and far more sickening than it appears on the surface. Tons of sweaty people cooped up, reusing the same dumbbells, handles, and machines over and over again. It's not like anybody washes their hands mid-workout. And guess what? Gyms are full of bacteria. In fact, most of your favorite stuff is coated in detritus and disease -- even more than most toilets.
According to a study conducted by Fitrated, a website that reviews fitness equipment, treadmills, exercise bikes, and free weights all have crazy-high levels of colony-forming units (CFUs), or bacteria that can cause you serious health problems. Those CFUs include gram-positive cocci, which can cause skin infections and pneumonia; gram-negative rods, which cause infections and may resist antibiotics; and gram-positive rods, which can cause infections; and bacillus, which can lead to ear, eye, and respiratory infections. Fun times!
The study tested 27 different pieces of equipment at three different chain gyms, so it's nowhere near exhaustive. Even so, treadmills and exercise bikes came out as the grossest, with 1.3 million CFUs per square inch, while free weights had 1.1 CFUs per square inch -- to wit, orders of magnitude worse than what you'd find on a toilet seat (3,200 CFU/sqin), a water faucet (18,000 CFU/squin), or a cafeteria tray (33,800 CFU/sqin). Yes, free weights have 362 times more bacteria on them than a toilet seat.