Two guests were recently diagnosed with Legionnaires disease as a Marriott hotel is suspected as the source of the bacteria.
The Missouri department of health and senior services said preliminary tests show evidence of legionella bacteria at the St Louis Marriott West Hotel.
State health officials are warning guests to seek medical attention if they have developed any symptoms in the past two weeks.
Possible symptoms include shortness of breath, fever, muscle aches and nausea.
The DHSS is still conducting further tests at the hotel.
Initial tests were carried out in the cooling tower, drinking water and showers.
The DHSS said it can be contracted from small water droplets with Legionella germs.
"The health and well-being of our guests and team members is our top priority. As soon as we were made aware of the situation, we fully cooperated with the health department to allow them to test and evaluate all of the hotel’s systems," a statement from hotel management said.
"We will continue to update everyone as the solutions are implemented and systems are re-tested for safety."