More than 4,000 delegates from the Pennsylvania chapter of the American Legion traveled to the Bellevue-Stratford Hotel in Philadelphia on July 21, 1976, for the Legion’s annual three-day convention. It was supposed to be remembered as the bicentennial commemoration of the 200th anniversary of the signing of the Declaration of Independence.

A medical mystery changed all that.

On July 27, three days after the convention ended, Ray Brennan, a 61-year-old retired U.S. Air Force Captain and an American Legion bookkeeper, had died from an apparent heart attack. By August 2, hundreds of Legionnaires had been sickened, and 22 were dead. In all, the case count eventually reached 221 ill and 34 dead.

It was one of the worst U.S. medical tragedies of the 20th century.

Dr. Ernest Campbell, a physician from Bloomsburg, PA, was the first to see a pattern in the outbreak after he realized that three of his patients with similar symptoms had attended the convention. He contacted the Pennsylvania Department of Health to report his findings. Officials at the American Legion also became concerned when notified of the sudden death of numerous members.

The Centers for Disease Control and Prevention (CDC) was called in and mounted one of its largest investigations ever to try to identify the cause. Bioterrorism, foul play, microorganisms, and toxins all were considered.

Five months later, after much speculation and failed tests, CDC microbiologist Joseph McDade finally identified a red, rod-shaped organism as the cause of the illness. It was a bacteria unlike anything scientists had seen before. They called it Legionella.

In April 1977, the CDC coined the name of the disease that had sickened and killed so many and baffled scientists for months as “Legionnaires’ disease.”

Illness caused by Legionella bacteria continues to be detected, now more than ever, according to the CDC. About 6,000 cases were reported in the U.S. in 2015, but because Legionnaires’ disease is underdiagnosed, the actual number of individuals infected each year is unknown.