The discovery came after the city switched its water supply and the medical staff noticed an increase in people coming in for treatment who were diagnosed with Legionnaires,' McLaren Hospital said.
Legionnaires' disease is a respiratory bacterial infection usually spread through mist that comes from a water source.
McLaren Hospital said it has taken corrective measures with its water supply.
All testing shows the hospital water supply "is well within safety and quality standards," the hospital statement said, and no tests show McLaren was the source of Legioinnaires' disease.
The hospital report is the latest negative news about the Flint water supply and the second piece of news about Legionnaires' disease
About two years ago, the state ordered the city to switch its water supply from Lake Huron to the Flint River as a money-saving measure.
After high levels of iron and lead were found in the water, authorities realized they had a crisis on their hands.
Residents began drinking bottled water and using water filters. The governor apologized to citizens. The National Guard was called in.
Then Gov. Rick Snyder announced a spike in the number of cases of Legionnaires' disease
in Genesee County as a whole in the two years since the Flint water supply switch.
From June 2014 to November 2015, at least 87 county residents developed Legionnaires' disease, compared to between six and 13 cases in the four preceding years, said Nick Lyon, director of the Michigan Department of Health and Human Services. Ten patients died, he said.
But Lyon said there's not necessarily a cause-and-effect and that it would be nearly impossible to say the water switch caused the spike, partly because not all the people who got Legionnaires' were exposed to Flint water. He said further testing will be ordered.
The hospital said it noticed an increase in Legionnaires' cases coming to McLaren, plus the communitywide increase, after the water supply switch.
The hospital began "aggressively" testing its water supply and an early test result indicated the presence of a low level of legionella, the statement said.
The hospital said it installed a secondary water disinfectant system throughout the entire facility at a cost of $300,000 and also installed lead filters on water and ice machines.
"It is important to note that no tests have ever determined that McLaren is the source of exposure for any patients testing positive for the legionella antigen, and that there is no definitive data to support that McLaren Flint is the source of exposure for any patient testing positive for the legionella antigen," the statement said.