In the midst of a star-spangled summer in which the United States celebrated its bicentennial, more than 4,000 members of the Pennsylvania chapter of the American Legion gathered just blocks away from Independence Hall where the country’s forefathers had severed their ties with King George III two centuries earlier. While Philadelphia sweltered on July 21, 1976, the military veterans discovered an icy refuge inside the air-conditioned quarters of the elegant Bellevue-Stratford Hotel as they kicked off the organization’s annual convention. For four days Legion members mixed and mingled inside the Philadelphia landmark, dubbed “The Grand Dame of Broad Street,” before returning home after what they believed was another successful gathering.

Within days, however, the phone at the American Legion’s Pennsylvania headquarters began to ring with the distressing news of the deaths of a number of convention-goers. By August 2, however, it was clear that this was no string of bad luck as 12 members had died and three dozen more had been hospitalized with a mysterious respiratory illness. The pneumonia-like symptoms were nearly the same in every case—muscle aches, headaches, severe coughs, diarrhea, muscle and chest pains and fevers as high as 107 degrees. Many of the dead were older men and smokers, but the ages of the victims ranged from 39 to 82.

As news spread, it was revealed that not all the afflicted were American Legion members or their wives. The victims included a bank teller who worked across the street from the Bellevue-Stratford Hotel and a bus driver who had transported a group of young cadets who marched in the convention’s parade.

Although baffled as to the cause, public health officials urged calm as fears of a flu pandemic spread around Pennsylvania even more quickly than the mystery illness itself. Swine flu, which had struck a New Jersey army base earlier in the year, and parrot fever, spread by sick pigeons, were among the leading theories. The good news for investigators, however, was that it quickly became evident that the disease was not contagious. One convention-goer, for instance, exhibited no symptoms although the two men he shared a hotel room with had died suddenly. Antibiotics also proved effective in treating the sick.

In response to the medical mystery, the federal Centers for Disease Control and Prevention (CDC) launched the largest investigation in its history. “No previous scientific detective effort in history has approached the scale and intensity of the campaign now under way to track down the course, source and pattern” of the disease, reported the Boston Globe. A team of 20 CDC epidemiologists joined state health workers in scouring hospital records and poring through autopsy findings. Laboratories remained open throughout the night as helicopters flew in the latest blood and tissue samples. In hospitals across Pennsylvania, the medical sleuths interviewed patients about their every move in Philadelphia, from whether they ate the hotel’s go-getters’ breakfast to how many times they rode its elevators.

Investigators even checked into the Bellevue-Stratford Hotel and combed the premises for clues. They examined everything from the hotel’s ice machines to its toothpicks, and they crawled into its heating and cooling systems to take samples. They considered causes ranging from food poisoning to foul play by anti-war protestors who had previously threatened violence against military veterans. The only common threads the investigators could find, however, were the disease’s symptoms and the fact that the afflicted appeared to have spent some time either in the hotel lobby or outside on the sidewalk.

The outbreak of the mystery disease generated intense media coverage. Newsweek called it the “Killer Fever,” while Time dubbed it the “Philly Killer” on its front cover. Most of the media, however, settled on another name for the strange respiratory illness—“Legionnaires’ disease.” As months progressed without the identification of a cause, the medical investigators themselves came under the microscope of public scrutiny—even being forced to testify before Congress.

One frustrated CDC microbiologist, Joseph McDade, decided to redouble his efforts in the days after Christmas. Having canceled his vacation plans, McDade spent hour upon hour in his laboratory scouring slides that had only been examined in five-minute bursts in the initial rush to find the cause. “It’s like looking for a contact lens on a basketball court with your eyes four inches above the ground,” McDade told the New York Times. After spending a half-hour examining tissue taken from the lung of one of the victims, McDade found the culprit for the disease—a previously unknown bacterium that the CDC dubbed Legionella.

Nearly six months after the outbreak, the CDC announced that it had cracked the case. The Legionella bacteria thrived in hot weather and in water such as the air-conditioning system perched on the roof of the 19-story Bellevue-Stratford Hotel. Although Legionella wasn’t found in the hotel’s cooling system because it had been cleaned by the time of its discovery, investigators surmised that the system’s powerful fans emitted a mist of contaminated water that fell on pedestrians on the sidewalk below and were sucked into the lobby through a ground-floor vent where victims breathed in the tiny, infected water droplets. Ultimately, 34 people died and more than 200 became ill from the outbreak during the American Legion convention, and the discovery led scientists to document earlier outbreaks of Legionnaires’ disease, including one that killed three members of the Independent Order of Odd Fellows who attended a convention in the same Philadelphia hotel in 1974.

Although the medical case had been solved, Legionnaires’ disease has not been confined to the history books. In fact, it has made a resurgence in recent years. According to the CDC, the number of people diagnosed has increased nearly fourfold from 1,127 in 2000 to 5,166 in 2014, with the disease proving fatal in about seven percent of cases. Just last year, an outbreak in the Bronx killed 16 while another in Flint, Michigan, claimed the lives of a dozen more. Most of the 20 outbreaks averaged each year occur in buildings with large water systems and poorly maintained air-conditioning equipment.