Legionnaires' disease has been in the headlines on both sides of the border this summer, with public health officials investigating outbreaks in Ontario and in neighbouring New York state.

An July outbreak in London, Ont., which has since been declared over, left three people dead and 70 infected. The source of the outbreak is not yet known.

A recent outbreak in New York City, meanwhile, has killed three people and sickened more than five dozen since late July. 

Health officials in that city link the Central Harlem outbreak to cooling towers — structures containing water and a fan that are used to cool buildings. They said 11 of these towers initially tested positive for a type of bacteria that causes legionnaires' disease but the problem has been remedied.

WATCH | A legionnaires' disease outbreak in London, Ont.: 

Legionnaires' outbreak leaves dozens sick in London, Ont.

An outbreak of legionnaires' disease, a serious respiratory illness, in London, Ont., has killed one person and made more than 35 people sick. Health officials do not know where the outbreak started or where it’s spreading.

What is legionnaires' disease?

Legionnaires' disease is a respiratory infection caused by the Legionella bacteria.

Early symptoms include fever, chills and a dry cough.

It can lead to a serious chest infection or pneumonia, with symptoms that may include high fever. The symptoms usually develop days after being exposed to the bacteria — and can be fatal.

The name of the disease comes from an outbreak of pneumonia that killed 29 people at an American Legion Convention in Philadelphia in 1976.

How does it spread?

Legionella bacteria live in various sources of water — both natural and man-made.

People can get infected after inhaling water droplets contaminated with the bacteria. That might be through breathing in droplets, or mist released to the air from things like cooling towers, hot tubs or plumbing systems.

Legionnaires' disease cannot be spread from one person to another, the Public Health Agency of Canada (PHAC) said.

Cooling towers, a component of industrial air-conditioning equipment, can be a good environment for the bacteria to grow, and these towers can release large quantities of water droplets into the air.

Since a cooling tower emits evaporated air, it could create conditions for water droplets contaminated with bacteria to be sent into the air and spread by wind, control experts say.

This is why they are often linked to outbreaks of legionnaires' disease, PHAC said.

Outbreaks occur more frequently during periods of warm weather, researchers have found.

Risk factors for developing legionnaires' disease include being older than 40 years of age, smoking, alcohol consumption, chronic lung disease, chronic kidney disease, diabetes, being immunocompromised and recent travel.

Is it a severe disease?

The infection can lead to pneumonia, with symptoms that may include high fever and chills.

Sometimes there can be gastrointestinal symptoms.

"By the time they come to the hospital, they're already very short of breath," said Dr. Zaki Ahmed, chief of staff at Toronto's Humber River Hospital and a critical care physician who has treated legionnaires' disease.

"They're having some chest pains, they're having nausea, vomiting. They may or may not have confusion," he said.

Because it's a rare disease, the mortality rates are difficult to estimate, Ahmed said.

How is it treated?

When patients come to the hospital with pneumonia, the antibiotics that doctors commonly prescribe typically kill Legionella, said Dr. Isaac Bogoch, an infectious diseases specialist based out of the University Health Network's Toronto General Hospital.

But the antibiotics won't be enough in some severe cases, he said.

"It's not just the infection, it's also the inflammatory response following the infection," Bogoch said.

In the most severe cases, multiple parts of the lungs can be impacted by the disease, and some people with the infection will need to be treated in the intensive care unit, he said.

What about prevention?

Public Services and Procurement Canada notes that Legionella bacteria can proliferate in building water systems under certain conditions:

• Temperature ranges between 20 C and 50 C.
• Stagnant water.
• Sanitation in the system is lacking.

Overall, the risk of getting legionnaires' disease is generally quite low, PHAC said.

"In your home, you can reduce the risks through proper maintenance of all mist-producing devices, such as shower heads, hot tubs, whirlpool bathtubs and humidifiers," the federal health agency suggests. "Make sure you clean and disinfect these devices regularly according to manufacturer directions."

But bigger buildings often use water as a cooling source, Humber River's Ahmed said. As they do, the cooling units aerosolize that water into the air, which we then inhale, he said.

Public Health Ontario said Legionella bacteria are "ubiquitous" in the environment and most cases in the province are sporadic.

Between 2018 and 2023, PHAC reported an average of 620 confirmed cases each year.

What's happened in previous outbreaks in Canada?

In 2012, an outbreak of legionnaires' disease in Quebec City caused 14 deaths and made about 200 people sick. Health authorities confirmed that an air-conditioning unit at the top of an office building in Quebec City was the origin of the bacteria. The building was owned by the Centrale des syndicats du Québec.

In response, the Quebec government introduced new regulations for the operation and maintenance of cooling towers, such as a registry and a certified control plan for each tower.

In New Brunswick, public health officials also recommended a cooling tower registry and associated rules to stem outbreaks.

A long-term care home in Toronto's east end was linked to 23 deaths in 2005.

That outbreak was traced back to water droplets from a cooling tower on the roof of the nursing home, city officials said at the time.