Thorough inspections are planned for almost two dozen cooling systems in buildings in the area where the outbreak is believed to have originated. The checks are expected to start on Tuesday.
Dr. Francois Desbiens, regional director of public health, said Monday the bacterial infection has resulted in 40 cases of the respiratory illness in the provincial capital since July. Two men and one woman have died.
The bacteria grow in stagnant water in the cooling units used in large buildings before circulating in the air-conditioning system.
It then passes into the air as fine droplets.
Inhalation of the droplets is generally not a problem for people in good health but can affect heavy smokers or those with weakened immune systems. Symptoms include a high fever and breathing problems.
Desbiens said there are usually two or three cases of legionnaires' disease every year.
The current group of patients is on average 62 years old and there are two men for every woman. Two men and one woman died.
"These are people who lived at home," Desbiens said of the cases.
"Most had a chronic illness or were heavy smokers. There are two or three younger cases who had no contributing risk factors except working in the area."
Letters have been sent to the owners of buildings of three storeys and taller, asking them to disinfect their cooling sytem.
Water samples will be taken during the inspections, which are expected to last until Friday.
"There have been interventions but despite this the problem continues," Desbiens said in outlining the planned actions. "So you have to take things up a notch."
While voluntary compliance with efforts to curtail the disease have been sought, city regulations allow inspectors to enter the buildings.
Chantale Giguere, assistant director of public security for the city, said the municipality had already cleaned its four buildings in the affected area.