In July 1976, thousands of veterans gathered for a meeting of the American Legion at the Bellevue-Stratford Hotel in Philadelphia.
A few days later, a retired US Air Force captain died of an apparent heart attack.
Within a week, another eight men were dead. More were sick with fevers, shortness of breath and chest pains.
"This was big news that these war veterans were being sickened," says Robert Promisloff, a clinical professor emeritus of medicine at Drexel University in Philadelphia.
The disease came to be called Legionnaires' disease, after the American Legions who fell ill.
Fifty years on, cases of the deadly disease appear to be rising around the world, and experts are not certain why.
The first outbreak
Professor Promisloff was training to become a lung specialist at Philadelphia's Hahnemann University Hospital when the first outbreak occurred.
"We were all scratching our heads," Professor Promisloff says.
"Nobody really knew what was going on."
Patients were admitted with pneumonia, but the usual antibiotics weren't working.
"That was extremely difficult … watching people get sicker and sicker," Professor Promisloff says.
In total, 182 people fell ill and 29 died.
More than 2,000 members of the American Legion attended the conference at the hotel. (Supplied: US Library of Congress)
The US Centers for Disease Control and Prevention (CDC) launched an investigation.
Experts knew the Bellevue-Stratford Hotel was at the centre of the outbreak, but they struggled to pinpoint the source.
Was it something the attendees ate? Were only certain rooms affected? The CDC even tested cigarettes given to participants in their conference materials.
"There were people picketing in front of the hospital, handing out leaflets that it was a government plot to kill people," Professor Promisloff says.
"Some newspapers were calling it a hoax. They were blaming the government. They were blaming the communists."
Conspiracy theories emerged as authorities struggled to find the cause of the deadly pneumonia. (Supplied: US CDC)
A lucky discovery
Meanwhile, CDC microbiologist Joseph McDade was trying to work out what caused the disease.
He had noticed occasional rod-shaped bacterium in lung samples taken from patients, but not enough to raise alarm bells.
Joseph McDade isolated and identified the bacterium that causes Legionnaires' disease. (Supplied: US CDC)
Dr McDade started looking for a virus instead, using antibiotics to knock out the rods "contaminating" the samples.
Six months on, he still hadn't found the culprit.
Legionnaires' disease was also called the "Philly Killer" when the outbreak began. (Supplied: US CDC)
Around new year's, Dr McDade visited his lab to declutter and decided to look at the samples again.
This time, instead of seeing a single rod-shaped bacterium, he saw a cluster he named Legionella pneumophila.
It took further outbreaks to discover the likely source of the Philadelphia outbreak was the hotel's water cooling tower that fed its air conditioning system.
Legionella lives in soil and water, and is known as an "accidental" pathogen because it doesn't need to infect humans to survive.
The bacteria are found globally in natural waterways in low concentrations, but causes problems when concentrated in human-built infrastructure.
It took six months for a CDC scientist to identify the bacteria responsible for Legionnaires' disease, which appears here in multiple circular and rod-shaped clusters. (Supplied: US CDC / Martin D Hicklin)
"If [Legionella] is in high numbers in a water cooling tower, and someone who's vulnerable breathes in the aerosol, it can colonise the lung," says Verlaine Timms, a microbiologist at the University of Newcastle and University of Western Sydney.
Most healthy people won't get sick. But it can affect people with weakened immune systems, such as older people and smokers, unless it's treated with specific antibiotics.
Australia's biggest outbreak
Australia's first-known case of Legionnaires' disease occurred in 1979, just a few years after the bacteria was discovered.
But the country's biggest outbreak happened in April 2000.
At the time, Jane Greig was training to become an epidemiologist.
She had been in Melbourne only a few months when Victoria's health department was notified that four people had contracted Legionnaires' disease.
All of them had visited the recently opened Melbourne Aquarium.
Melbourne Aquarium was at the centre of a Legionnaires' outbreak in April 2000. (ABC News)
"I was in the office of my field supervisor when someone came in to tell him they were going to send an investigation team to the aquarium, and I was offered, 'Do you want to go along?'" Dr Greig recalls.
The team took samples from all the fish tanks, but strongly suspected the aquarium's water cooling towers.
Days later, samples from the cooling towers tested positive for Legionella.
The health department set up a 1300 telephone number for concerned members of the public.
"That took over the boardroom on our floor of the department, and there was maybe 10 staff at any one time taking calls well into the evening," Dr Greig says.
In total, 125 cases were confirmed, and four people died.
Melbourne Aquarium stopped using water cooling towers as part of its air conditioning system. (ABC)
After the 2000 outbreak, the Victorian government introduced stricter rules on water cooling towers, bringing it in line with New South Wales, which had cracked down in response to major outbreaks years earlier in Wollongong and Fairfield.
Regulations differ across Australia's states and territories, but typically involve installation requirements and regular inspections.
Cases on the rise
Although we now know the culprit, Legionnaires' disease has surged over the past three decades.
New York is currently dealing with its second deadly outbreak this year. Eleven people died in the first cluster, and the latest outbreak has killed two.
Australia has so far avoided a major outbreak in 2026.
But data suggests cases are trending up, particularly in NSW and Victoria.
"There's a couple of reasons that people believe contribute," Dr Timms says.
Big buildings like offices and hospitals typically use cooling towers in their air conditioning systems. As cities get bigger, the number of water cooling towers increases.
Australia also has an aging population that's more likely to fall ill after inhaling Legionella.
Climate change has also been suggested as an explanation, given the bacteria thrives in wet, humid places.
But Dr Timms's research suggests this isn't the main driver of major outbreaks.
She hopes more thorough genetic sequencing of Legionella in water cooling towers will offer hints about why some bacterium cause such devastating outcomes.
"I don't think you can ever control all Legionella. They are everywhere and they are very well adapted," Dr Timms says.
"But you want to be able to control the Legionella that sporadically causes us problems."
Hear more about Australia's biggest Legionnaires' outbreak on Science Friction: Outbreaks on ABC Radio National.