Public health officials say the likely source of that outbreak is the hotel's main fountain in the lobby.
"We took it upon ourselves to actually remove the fountain out of the lobby," said Catherine Mrowiec, general manager, JW Marriot.
The Chicago Department of Public Health says it now has 10 confirmed cases of the disease.
Guests who became ill stayed at the hotel from July 16 through August 15.
For most patients, the symptoms of Legionnaires are mild and include headache, fever, chills and cough. But for some patients, especially those with respiratory distress, it can be more serious and sometimes fatal for those who develop pneumonia. Doctors say it is almost always is spread through aerosolized water from fountains, showers, hot tubs or other sources.
"The bacteria is everywhere. In muddy water, the ground, soil ... once it gets into the atmosphere, it gets aerosolized and you get enough of it up in there , that's when it becomes a problem," said Dr. Mark DeSilva, Gottlieb Hospital.
Decorative water fountains can even be a problem in typically healthy environments like hospitals.
In a news release, the health department says, "There is no ongoing risk to Chicagoans."