A spokesman for the Dupage County Health Department confirms that two men have contracted Legionnaires Disease.
The Legionella bacteria has been traced back to a hot tub at the LA Fitness on Freedom Drive in Naperville after the incident was first reported on October 23.
Health department spokesman Jason Gerwig described the two cases as "pretty unique," and said tests on the shower and the pool at the health club came back negative.
The victims were listed in stable condition.
The health club, minus the hot tub, remains open while testing is done on the area, but gym members tell NBC 5 that they weren't told there was a Legionnaires disease issue in the building.
"That's something they have to be more careful and let the people know," gym member Ricardo Sequeria said.
Legionnaires disease, a severe form of pneumonia, is typically contracted by breathing in a mist or vapor that contains the bacteria. The symptoms typically include coughing, high fever and chills.
It's not considered life-threatening, but complete recovery can take up to four months.