Authorities suspect the cause might be the air-conditioning system in a large public building — although they haven't identified the building yet.
The regional public-health authority has sent a notice to the owners of all large buildings within a 1.5-kilometre area to clean their cooling towers.
It believes the bacteria that causes Legionnaires' developed in the stagnant water in one of those towers, connected to the air-conditioning system.
Legionnaires' disease is contracted by breathing in small droplets of water contaminated with the Legionella bacteria. The symptoms are similar to those of flu, including coughs, fever and chills.
The disease is not contagious and cannot be transmitted from one person to another. It presents little or no risk to most people, although elderly people are more vulnerable.
Radio-Canada reports that the victim is an 88-year-old woman.