On this day in 1976, members of the American Legion arrive in Philadelphia to celebrate the bicentennial of U.S. independence. Soon after, many began suffering from a mysterious form of pneumonia. Their ailment would come to be known as Legionnaires’ disease.

About 4,000 delegates from the Pennsylvania chapter of the American Legion met at the Bellevue-Stratford Hotel in Philadelphia for a four-day gathering. While at the hotel, built in 1900, the Legionnaires did not notice anything unusual. However, several days after the event ended, many attendees became sick. By August 2, 22 people were dead and hundreds connected to the gathering were experiencing pneumonia-like symptoms.

The Center for Disease Control immediately launched an investigation, but it took four months to identify the culprit. Joseph McDade, a CDC research microbiologist, finally isolated the bacteria that caused the disease: an aquatic microorganism, found in watery places like pipes and air conditioning units, which caused a low fever and mild cough in most people who were exposed to it, but could affect other people in far worse ways. In a small, but significant, minority of people, vomiting, diarrhea and pneumonia developed, following an incubation period of between two and 10 days. Smokers, very old people and those suffering from pulmonary disease were most at risk.

From the American Legion event in Philadelphia, about 250 cases were identified, which resulted in between 29 and 34 deaths. Researchers estimate that there are about 20,000 cases of Legionnaires ‘ disease annually in the United States, but only about 1,000 are correctly identified and diagnosed, as its symptoms can be similar to regular pneumonia. Antibiotics are usually effective against the disease.

Scientists are still unclear as to how long Legionnaires’ bacteria had been striking victims before it was finally identified in 1976.