Mysterious Legionnaires’ Disease Strikes Summary

  • Last updated on November 10, 2022

More than two hundred people attending an American Legion convention at the Bellevue-Stratford Hotel in Philadelphia, Pennsylvania, were stricken with a mysterious lung disease. Investigators from the Centers for Disease Control ultimately discovered a bacterium responsible for the disease in the hotel’s air conditioning system, leading to worldwide tightening of sanitation standards for large cooling systems.

Summary of Event

From July 21 through July 24, 1976, the Pennsylvania chapter of the American Legion held its fifty-eighth annual convention at the Bellevue-Stratford Hotel in Philadelphia, Pennsylvania. Twenty-five hundred Legionnaires along with more than two thousand family members and guests attended the convention, a huge celebration of the two-hundredth anniversary of the signing of the Declaration of Independence. By the second day of the event, several veterans were complaining of influenza-like symptoms, and several developed acute pneumonia. Within weeks after the close of the convention, the Pennsylvania Department of Health identified the American Legion Convention and, in particular, the Bellevue-Stratford Hotel as the common link among the some 221 cases of the mysterious and serious disease that eventually killed 34 patients. Although not all victims had attended the convention, all had either been in or near the Bellevue-Stratford Hotel in late July. Diseases;Legionnaires’ disease Legionnaires’ disease Centers for Disease Control;Legionnaires’ disease [kw]Mysterious Legionnaires’ Disease Strikes (July 27, 1976) [kw]Legionnaires’ Disease Strikes, Mysterious (July 27, 1976) [kw]Disease Strikes, Mysterious Legionnaires’ (July 27, 1976) Diseases;Legionnaires’ disease Legionnaires’ disease Centers for Disease Control;Legionnaires’ disease [g]North America;July 27, 1976: Mysterious Legionnaires’ Disease Strikes[02490] [g]United States;July 27, 1976: Mysterious Legionnaires’ Disease Strikes[02490] [c]Health and medicine;July 27, 1976: Mysterious Legionnaires’ Disease Strikes[02490] Sencer, David J. Shepard, Charles C. McDade, Joseph Dowdle, Walter Fraser, David

People suffering from the illness reported initially feeling as though they had the flu: They complained of headaches, low-grade fevers, muscle pain, and loss of appetite. Victims next developed diarrhea and nausea, followed by a sharp upward spike in fever. Victims also developed a dry cough, and in the most serious cases, acute pneumonia and delirium. Although 221 people eventually sought treatment for the illness, it is unknown how many of the Legionnaires and visitors to Philadelphia suffered minor cases of the disease. More men than women fell ill, and those who had compromised immune systems, such as the elderly, alcoholics, and smokers, were most at risk for death.

On August 2, 1976, at the request of the Pennsylvania Department of Health and after the deaths of some twenty people, David J. Sencer, director of the Centers for Disease Control (CDC) in Atlanta, Georgia, called in researchers to begin an investigation into the cause of the disease and its means of transmission. Without an understanding of the source of the disease, medical personnel were not able to treat the illness effectively, and, without an understanding of how it was transmitted, public health officials were not able to adequately protect the population from falling ill.





The CDC researchers, including Walter Dowdle, Joseph McDade, and Charles C. Shepard, among others, quickly ruled out influenza as the cause of the disease, an important discovery because the nation was also bracing for a predicted deadly outbreak of swine flu. They also found that the disease was not spread from person to person. However, although the investigators were able to rule out influenza as the cause of the disease, they were not able to ascertain what the cause was. They tested lung tissue from dead Legionnaires but were not able to find a microbe by using standard procedures and dyes. They thus suspected a virus rather than a bacterium; however, when they injected samples of the diseased lung tissue into live chicken eggs, the chickens continued to grow healthily, suggesting that the disease was not a virus, either. The researchers next tested for rickettsia—very small, viruslike organisms that are actually bacteria—but the tests failed to demonstrate that they were the source. The investigators then thought that the causative agent could be a toxin or poison rather than a bacterium or virus, but leading toxicologists could find no link.

Finally, in December of 1976, McDade returned to his slides of lung tissue to reexamine some rod-shaped cells he noticed on them. When he tested the cells with blood from survivors of the disease, the antibodies in the blood attached to the rod-shaped cells. Shepard and Dowdle joined in verifying the results. They had found the source, a strange and previously unidentified organism researchers named Legionella pneumophila, Legionella pneumophila the causative agent of legionellosis, or Legionnaires’ disease.

It was little wonder that the researchers had been unable to identify the bacterium sooner: L. pneumophila did not take up stain; nor would it grow, except in the presence of large amounts of iron and cysteine. It consequently had remained invisible or inert under standard testing procedures. In addition, unknown to the researchers, mice were immune to the disease. Consequently, using mice as their test animals further misled the researchers. Once Shepard and McDade switched to guinea pigs, their efforts paid off.

Although researchers had identified the bacterium that caused the disease, they still did not know its ultimate source nor the means of transmission. They knew only that the Bellevue-Stratford Hotel was the source of the infection. Fortunately, a worker at the CDC recalled a similar, although less serious, outbreak of a disease at the county health department building in Pontiac, Michigan, that had occurred some eight years earlier. When frozen blood samples were tested, it was confirmed that “Pontiac fever” was actually legionellosis. Because researchers suspected that the air-conditioning system in Pontiac was the means of transmission, it gave them a place to start investigating in Philadelphia.

In January of 1977, investigators found L. pneumophila in the cooling tower of the air-conditioning system of the Bellevue-Stratford Hotel. At the time, it was believed that the air conditioning spread the bacterium throughout the hotel, infecting the Legionnaires and their families when they breathed in the contaminated air. This finding led to worldwide regulations concerning large-scale air conditioning units. Virtually all large buildings, including hospitals and hotels, as well as cruise ships, have since had to be guarded against legionellosis by having their air conditioning carefully monitored.

However, in recent years, researchers have suggested that there are many other ways that legionellosis can be spread, all involving water. The bacterium has been found in vegetable misters at grocery stories, hot tubs and whirlpool baths, and showerheads and fountains. It has also been found in hot springs and slow-moving or stagnant water. The bacterium is resistant to chlorine, so chlorinated water does not eliminate the danger.


In retrospect, researchers realized that Legionnaires’ disease was not truly a new disease in 1976; numerous outbreaks of pneumonia of unknown etiology had been identified as far back as 1947, and it is likely that the bacterium has existed for centuries. Researchers did, however, realize that legionellosis is a disease of the modern world, of large buildings where people come in contact with large-scale ventilation and water systems. Scrupulous attention must be given to these areas, and the disease has changed the ways buildings are constructed and monitored.

During the first part of the twentieth century, people had come to believe in humanity’s ultimate victory over disease. With the eradication of smallpox, the control of tuberculosis, and the reduction of malaria, confidence grew in the medical community’s ability to handle any medical crisis. Legionnaires’ disease, coming as it did at the bicentennial of the United States, and at the same time as the predicted swine flu epidemic, was a grim reminder that viruses and bacteria continue to find new ways to circumvent the “cures” humans develop for their eradication. If anything, Legionnaires’ disease demonstrated that humanity has only reached an uneasy truce with disease-causing entities in the modern environment, not a victory, and that many battles remain to be fought. Diseases;Legionnaires’ disease Legionnaires’ disease Centers for Disease Control;Legionnaires’ disease

Further Reading
  • citation-type="booksimple"

    xlink:type="simple">Garrett, Laurie. The Coming Plague: Newly Emerging Diseases in a World out of Balance. New York: Farrar, Straus and Giroux, 1994. Readable narrative account of many modern diseases, including a chapter covering Legionnaires’ disease.
  • citation-type="booksimple"

    xlink:type="simple">Thomas, Gordon, and Max Morgan-Witts. Trauma: The Search for the Cause of Legionnaires’ Disease. London: Hamish Hamilton, 1981. Dramatic and comprehensive history of the outbreak in Philadelphia. Draws on firsthand accounts of survivors and victims to create an immediate, exciting story.
  • citation-type="booksimple"

    xlink:type="simple">Zimmerman, Barry E., and David J. Zimmerman. Killer Germs: Microbes and Diseases That Threaten Humanity. Chicago: Contemporary Books, 2003. Excellent, up-to-date description of the origins and history of diseases, including a good section on Legionnaires’ disease. Also includes a comprehensive bibliography of books, articles, news accounts, and Web sites as well as a list of suggested readings.

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Categories: History