Health authorities in the Canadian city said that the death toll from an outbreak of Legionnaire's disease in Quebec City has risen to 12 since late July.
Officials said that 176 people have so far been diagnosed with the disease, which poses a risk for people with weak immune systems but can be treated with antibiotics.
Officials suspect improper maintenance of air conditioning systems as the cause of the outbreak.
Legionella bacteria grow in stagnant water, then spread as droplets are expelled by the appliances.
The illness, which has an incubation period of between two and 10 days, is an infection that causes high fever, dry cough and pneumonia.
The disease was discovered in the United States in 1976 during a convention of the American Legion, an military veterans group, where 29 people died.