The news that Legionnaires’ disease has killed a dozen New Yorkers this summer—and that an inmate at Rikers Island has been diagnosed as well—has put the illness in the headlines and hospitals. Luckily, these days, most people who experience symptoms can be treated successfully with antibiotics.

But that wasn’t the case when the disease was first discovered. Not only did Legionnaires’ prove fatal to many affected by its first notable outbreak, but those victims didn’t even know what was making them sick.

The legionnaires after whom the disease is named were attendees at an American Legion convention in Philadelphia in July of 1976. They were veterans like Ray Brennan, Frank Aveni and Charles Seidel, who went to an ordinary meeting, fell ill shortly after with chest pains and high fevers, and died not long after. They had spent time all over the city, eaten in restaurants and traveled home in a variety of ways. In a cover story that August about the “disease detectives” who were trying to find “the Philly killer,” TIME captured the panic that resulted from the outbreak. The magazine noted many friends and neighbors of John Bryant Ralph, another Legionnaire, did not attend his funeral for fear his disease might be catching.

For many Americans, the Philadelphia outbreak was the first time they were aware of the work done by the Centers for Disease Control, a then-obscure organization that went to work trying to figure out what was killing the Legionnaires:

Like police work, most medical sleuthing is done in the field by the “shoe leather” epidemiologists, some from the state’s public health service, others from the CDC. They crisscrossed the state to interrogate every one of the stricken Legionnaires and the families and friends of the deceased. Their quest: a common denominator, a set of experiences that would link all the victims, such as meals taken together, rooms in the same hotel, exposure to similar contamination. Their method: careful questioning and cross-referencing. “Hello, I’m part of the medical team investigating this weird disease,” said Dr. Stephen Thacker, 28, as he sat down beside Thomas Payne’s bed in Chambersburg Hospital. “How do you feel? When did you first feel sick? Where did you eat and stay in Philadelphia? When did you arrive there? When did you leave? Did you go to the testimonial dinner? Or the go-getter’s breakfast? Did you go to the hospitality rooms for the state commander or other officials? Did you have any contact with pigs?” In some ways the detectives’ legwork raised more questions than it answered. A check of the hotels at which the conventiongoers had stayed revealed no outbreak of the mysterious illness among employees who had come in contact with the Legionnaires. The investigators could find no evidence that any of the victims had been exposed to pigs, which have been implicated as the animal reservoir for the swine-flu virus. Nor could the disease detectives explain another apparent contradiction: why some people developed the disease, while others, who ate the same meals, drank the same drinks or shared their rooms during the convention, did not.

The disease remained elusive. “There’s an outside chance we may never find out the cause,” CDC Director David Sencer told TIME. “I think we will. But there are times when disease baffles us all. It may be a sporadic, a onetime appearance.”

Eventually, the initial outbreak tapered off and, though nearly 200 had become ill and 29 had died, fear tapered off too. It was clear that whatever it was, it wasn’t spreading—but, though public interest waned, the CDC kept working. It wasn’t until months later that the mystery was actually solved.

“Five months after the convention, [CDC microbiologist Joseph McCade] took another look at some red sausage-shaped bacteria and concluded that they were the culprits,” TIME later explained, in a 1983 cover story about the CDC. “They had festered in the water of the hotel’s cooling tower and had been carried through the air as the water evaporated.”

Read the 1976 cover story, here in the TIME Vault: Tracing the Philly Killer

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Write to Lily Rothman at lily.rothman@time.com.