The Chicago Department of Public Health and the JW Marriott Chicago Hotel issued an advisory Tuesday. They say guests with symptoms who stayed at the Adams Street hotel should contact their doctors.
Symptoms include headache, high fever, chills, cough, chest pain and shortness of breath.
The city and the hotel are notifying the 8,500 guests who stayed there from July 16 through Aug. 15. Authorities say they've identified the source of the bacteria and there's no ongoing health risk.
The bacteria that cause Legionnaires' disease grow in water and can spread through vapor in air-conditioning ducts or mist from a whirlpool spa.
A hotline has been set up by CDPH to answer questions from people who may have been exposed. That phone number is (312) 746-4835 during Monday-Friday from 8:30 a.m. to 4:30 p.m.
"We really are trying to be transparent and take this very seriously. And it's very important for us, and we want our guests to be safe," said Catherine Mrowiec, JW Marriott general manager.
The illness, a severe form of pneumonia, is contracted by inhaling bacteria in water vapor. It cannot be spread person to person and most people who are exposed don't become sick.
"People who are elderly, have weakened immune systems, people who have chronic lung disease are more at risk," said Dr. Kathy Ritger, Chicago Department of Public Health.
Symptoms usually appear quickly.
"Most people who are exposed don't get the illness," said Dr. Ritger. "And if it's already been a couple weeks since their stay, and they haven't developed any of these symptoms, like fever or cough, there's no worry at all.
Public health officials say Chicago usually averages about 30 cases of Legionnaires' disease a year.
In 2008, at least two people fell ill after staying at this hotel in McHenry. And in 2009, a CTA worker died from Legionnaires' disease after being splashed with water from a machine used to wash train cars.
JW Marriott guests say they were told of the situation at check-in.
"I was given the letter. I was allowed to read through that letter before I checked in and given the opportunity to ask questions," said Ian Evans, J.W. Marriott guest.
The Associated Press contributed to this report