Two more people are dead, amid of a total of 40 infections, in an outbreak of legionnaires' disease in Quebec City, public health officials said Monday.

A microscopic image from the U.S. Centers for Disease Control shows a large grouping of Legionella pneumophila bacteria, the pathogen behind legionnaires' disease. (Janice Haney Carr/Associated Press)

That's more than twice the number of cases that had previously been disclosed, and surpasses the last outbreak in the city, in 1995, when 12 people fell ill and one died.

Officials didn't name or provide details about the two new fatalities, which bring the death toll to three. The first to die was an 88-year-old Quebec City woman.

Dr. François Desbiens, the director of public health for the Quebec City region, said authorities have begun inspecting downtown buildings that use a cooling tower as part of their ventilation.

Public-health officials had already sent 2,700 letters to building owners in the downtown core asking them to clean their cooling systems, but the emergence of new cases — a dozen since Friday afternoon — is prompting the inspections.

"We're still seeing cases despite the fact that we've asked building owners to take steps," Desbiens said. "So we've decided to go further and investigate every tower again, and for those that haven't had maintenance, to identify them, to inspect them to take a water sample. And to order a disinfection."

Bacteria spread in mist

Legionnaires, also known as legion fever, is a flu-like pneumonia that affects people with lung problems or weak immune systems. It is caused by bacteria that thrive in warm temperatures and can breed in the stagnant water of cooling towers, before spreading in the mist released by those systems.

It's not contagious and presents no risk to people in good health. The disease got its name in the 1970s after an outbreak of pneumonia among people attending a convention of the American Legion.

Earlier this month, Desbiens guessed there were about 200 cooling towers in the area where people became sick that could be the source of the infection, but officials said Monday they've narrowed their area of concern down to 22 cooling towers in a core area that will be inspected by the end of the week, and 19 more in a different area that will be inspected after.

Tests on the water in those towers will take two to three days to get results confirming an infection with legionella bacteria.

There are normally two to three isolated cases of legionnaires' disease a year in the Quebec City region.