Some bacteria, fungi and viruses thrive in the warm, sweaty Petri dish of athletic gear, yoga mats and water bottles.
Without proper laundering, the germs may accumulate and find their way into your system, where they can cause gastrointestinal disease, according to Ottawa microbiologist Jason Tetro.
If you head into the shower at a gym with bare feet, "there's a good likelihood that you might be coming out of there with either a wart or athlete's foot," Tetro said.
At his lab, Tetro tested a gym bag, bottle and mat for CBC News. The bag was relatively clean but the water bottle contained bacteria from backwash that could grow to levels he said could potentially cause an illness within 24 hours.
"Just wash it out with some good hot water and maybe some detergent," Tetro suggested.
The mat was the most contaminated item.
"The mats themselves are incredibly porous and they're going to be able to keep in anything that you've put on there like sweat," he said.
Sports teams have had players come down with community acquired methicillin-resistant Staphylococcus aureus or MRSA, which can lead to illness or large boils on the skin.
Anaerobic bacteria that don't need oxygen to survive can live in the moist crevices of equipment, where they can grow and start to stink.