Officials say the men contracted the bacteria from a hot tub. That hot tub is off-limits until tests reveal it's no longer contaminated.
Legionnaires' disease is a severe case of pneumonia, essentially caused by bacteria. DuPage County health officials say the two men are medically stable.
"They both had been at this fit club, and they both were in the hot tub. And so we worked with LA Fitness and tested the hot tub, and we discovered that was the source," said Jason Gerwig, DuPage County Health Department.
DuPage health officials say tests show the pool and showers are safe.
"I would be a little concerned. I haven't been in the hot tub, so I'll probably try to stay away from that. I'll keep lifting weights," said Carl Leitelt, LA Fitness patron.
To reopen the hot tub, there must be two negative tests, and that, health officials say, may be a challenge.
"This disease can be really get underneath the slime of a pipe or something. You can throw bleach on that, but that doesn't mean that the bleach is penetrating it. You really have to get in there," said Gerwig.
Legionnaires' disease, which is in water droplets, has proven deadly. Three people died last year after contracting the disease at the downtown JW Marriott hotel. Health officials suspected the fountain in the lobby; hotel officials had it removed.
"You can't expect it to be like a soup kitchen, squeaky clean. When you come to the gym, there are bound to be risks," said Jonathan Ille, LA Fitness patron. "I would like to use it in the future. It doesn't seem like it will be a big deal for me."
The hot tub is expected to be closed for at least a month while it is cleaned. LA Fitness did not return ABC7 Eyewitness News calls or e-mails for comment.