Authorities suspect the cause might be the air-conditioning system in a large public building — although they haven't identified the building yet.
So far, 15 people have been infected in the capital this summer. There are usually two isolated cases of the disease per year.
The disease breeds in stagnant warm water, the likes of which are often found in large cooling systems. People get sick when they breathe in infected mist or water droplets.
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.
Doctor François Desbiens from the Quebec Public Health Agency said finding the original water source is often difficult.
"We know from previous eclosions in the United States and Europe that, with the wind, the droplets can go as far as one kilometre away from the building they are coming from."
The agency has sent 2,500 notices to the owners of all large buildings within a 1.5-kilometre area to clean their cooling systems.