Four decades ago it was one of medicine’s greatest mysteries, devastating a group of convention goers with shocking speed and leaving scientists scratching their heads about its cause.

Now Legionnaires’ disease is back in the news, having killed seven and sickened more than six dozen others in New York, with more diagnoses likely on the way.

Jay Varma, deputy commissioner for disease control in the city’s health department, told the Associated Press that disease-causing bacteria have been found in five cooling towers around the South Bronx. People who inhaled contaminated water droplets or vapor from the towers before they were cleaned are still at risk for infection, he said.

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Wary of the illness, residents have taken to drinking bottled water and are criticizing city officials for not reacting to the outbreak more quickly and monitoring water towers more closely.

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These days Legionnaires’ causes between 8,000 and 18,000 hospitalizations a year in the U.S. It’s treatable with antibiotics and usually not deadly except in those with underlying health problems. All of those killed in the Bronx outbreak had pre-existing medical conditions, officials said.

But Legionnaires’ has a fearsome reputation that lingers from the strange circumstances of its first documented diagnosis 39 years ago, in a case that scientists called “the greatest epidemiological puzzle of the century.”

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Nothing seemed amiss when hundreds of members of the American Legion gathered at the swanky Bellevue-Stratford Hotel in Philadelphia during the muggy days of late July 1976. They met, paraded and hobnobbed at cocktail parties, and then they went home, apparently without incident.

What happened next came all too fast.

Dozens of convention-goes were abruptly felled by intense, flu-like symptoms: wracking coughs, painful chills and fevers as high as 107 degrees. In the worst cases, victims’ lungs were flooded with a frothy, bloody fluid that filled the spaces between air sacs and prevented oxygen from getting into their bloodstream. The victims with this raging form of pneumonia almost always died.

Early news reports called the outbreak “explosive.”

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“The disease struck with unusual suddenness and killed so swiftly that a number of persons had died before the state health department even knew it had an outbreak on its hands,” read a front page article in The Washington Post from Aug. 3, 1976, less than two weeks after the convention.

By that point, 20 people had been killed and 115 other hospitalized by the mysterious disease. Pennsylvania health officials had only been informed of the outbreak the previous day, when state Legion officers called them up to report the alarming number of members who had fallen ill. By the time the disease had run its course, 221 people would be infected and 34 dead — all of the victims somehow connected with the gathering at the Bellevue-Stratford.

At the time, health officials didn’t know that the disease would be contained to convention-goers. They didn’t know that the outbreak had a course to run. Many feared that the illness was viral and could spread to hundreds, even thousands of others.

The Centers for Disease Control dispatched a squad of 20 epidemiologists — the largest in the agency’s history, according to the New York Times — to Pennsylvania to investigate the cause of the deadly illness. But they were utterly mystified by what they found there.

Aside from their attendance at the Legion convention, there were no common denominators among those felled by the illness. They had not eaten at the same restaurants or slept at the same hotels. They were all men, and mostly middle aged or older, but that seemed more a product of the American Legion’s membership than a trademark of the disease. Hotel employees seemed mostly unaffected by the disease, but a few people with only glancing connections to the convention —  a man who drove a bus of Legionnaires to the convention — also fell ill.

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Despite initial fears, the infection didn’t spread to the intimate acquaintances of those who were sickened, such as spouses or roommates. But the worsening condition of those infected kept officials concerned. Their symptoms didn’t precisely match either bacterial or viral forms of pneumonia, and while some victims responded well to antibiotics, others showed no improvement.

“We thought we might be faced with an unprecedented condition in modern medicine, one for which we had no really effective antibiotics, drugs or therapy,” Leonard Bachman, the Pennsylvania health secretary, told a New York Times reporter at the time.

The film “The Andromeda Strain,” based on Michael Crichton’s bestseller about a deadly alien virus, had come out just five years earlier, stoking America’s anxieties about the possibility of deadly diseases. To complicate things further, the outbreak came as the federal government mulled mandatory vaccinations for a new strain of influenza — swine flu — that public health officials warned could lead to a devastating epidemic.

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“If it’s swine flu, then we’re finished.” That’s what Robert Sharrar, a communicable disease specialist in Philadelphia’s public health department, thought on the morning of Aug. 2, as he read the reports about the illness.

“We didn’t have enough time. We didn’t get everyone immunized,” he recalled thinking in a December 1976 interview with the New York Times. “The whole country, the whole world was going to get it.”

Even researchers who had nothing to do with the investigation were transfixed by the mysterious “Legion’s disease.” A Washington Post reporter who attended a weekly dinner of Johns Hopkins scientists found the conversation dominated by speculation about the cause of the illness. With few leads to guide them, even the wildest theories thrown out by participants — a madman with a squirt gun, a comic book villain dropping toxic pills into the hotel air vents — were given due consideration.

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“The more you think about it, the more nothing really seems reasonable,” Richard Traystman, a specialist in lung physiology, told The Post at the dinner.

Beyond academic circles, the theories were even more outlandish. An official of the Veterans of Foreign Wars claimed that the disease was a left-wing plot targeting veterans’ groups. Despite repeated official denials, rumors circulated that an Army chemical warfare truck had been parked in downtown Philadelphia during the convention, and that small portions of poison were missing from the military’s stock of germ warfare weaponry.

After a few weeks, tests confirmed that the illness was not a form of the influenza virus, a relief to health officials who feared it might spread further. But they were no closer to figuring out what it was, and the public was starting to notice.

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Officials spent many days chasing a theory that the lethal agent may have been nickel carbonyl, prompted by test results that revealed unusual amounts of the toxic compound in autopsy tissue and — a congressional investigation later revealed — a letter from a “demented veteran or a paranoid anti-military type” claiming responsibility for the deaths via the deadly substance.

Later, researchers were embarrassed to realize that the high levels of nickel were caused by contamination from the instruments used during autopsies. Chastened, they ordered pathologists to use plastic knives instead.

Well into the fall and no closer to figuring out what had caused the outbreak, CDC officials were summoned before Congress to explain their handling of the investigation. By winter, many said that health experts had dropped the ball on the illness. Most believed its cause would never be known.

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But those comments irked CDC microbiologist Joseph McDade, who remained “haunted” by the feeling that some small aspect of the illness had been overlooked, he told the New York Times in 1977. So, in the down time just after Christmas he returned to his old slides containing tissue from guinea pigs that had been inoculated with material from the Legionnaires’ victims.

For a full half hour he examined the slide bit by bit, looking for anything out of the ordinary.

“It’s like looking for a contact lens on a basketball court with your eyes four inches above the ground,” he said.

That’s when he found it: a tiny-cluster of rod-shaped objects, stained red by the chemical added to slides to make bacteria stand out.

Excited, he started to experiment further. The bacteria weren’t of any species known to science, but that made sense — until that summer, the disease hadn’t been known to science either. He tested specimens in his slides against tissue samples from some of the Legionnaires and his theory was confirmed: The same microbe was also evident in the vast majority of the victims.

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Later study found that the bacteria had been breeding in the water of a cooling tower for the Bellevue-Stratford Hotel’s air conditioning system. The convention-goers had likely inhaled the microbe-laden vapor, and older, less hardy ones had fallen ill.

Further researched revealed that the disease was not the terrifying new illness it had initially seemed — it had been around for years, silently infecting Americans without being noticed. Eventually, scientists discovered that the microbe had also been the cause of a smaller, previously unexplained outbreak at the same hotel two years earlier, as well as several other incidents. In fact, McDade later realized that the pathogen had been isolated as early as 1947, and just never given much study.

But when the bacteria was finally pinned down and classified, it was named in honor of its most famous victims: Legionella pneumophila. 

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