The latest cases were reported Thursday by public health authorities.
"We're very worried by the current situation," said Francois Desbiens, director of public health for the Quebec City region.
"It's the biggest outbreak of legionnaires' that Canada has seen in years."
The number of reported deaths has doubled, from three, in a few days. However, officials expressed hope that things might soon improve.
They said thorough inspections have been completed in 28 buildings in two different neighbourhoods, near the provincial legislature. The process of checking, and cleaning, the cooling towers in those areas was supposed to have been completed Friday but it ended faster than scheduled.
"We disinfected and cleaned all the towers simultaneously," Desbiens said.
"We should be able to see the situation correct itself in five to six or seven days. I'm making that estimate because the incubation period is two to five days.
"It is very possible that we will have other cases of legionnaires' (Friday), Saturday, Sunday, Monday and Tuesday. These would be people who were contaminated before we managed to disinfect the towers."
The bacteria grows 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 it can affect heavy smokers or those with weakened immune systems. Symptoms include a high fever and breathing problems.
Authorities say there are usually two or three cases of legionnaires' disease every year.
They still do not know which building is the origin of the outbreak, although they expect to have a better idea within a month from samples taken in recent days.
The municipal government expressed dismay over the cleaning standards in some of the buildings.
"We saw it all" during inspections, said Chantale Giguere, assistant director general for public security in Quebec City. She said she wants to see a provincial law that would force building owners to better maintain cooling towers.
Mayor Regis Labeaume, who was on holiday, said he was returning early and was expected to meet with reporters Friday morning.