On this day in 1977, Dr. Joseph E. McDade discovered the cause of a mysterious flu-like illness that had killed 29 people at an American Legion convention the previous year.
The epidemiological effort was conducted under intense media and political pressure. A swine flu scare earlier in the year had led the Ford administration to start a program of mass inoculation, which it then suspended after reports of severe side effects. Toxic chemicals were blamed, particularly after nickel from autopsy scalpels was mistaken for a causative agent. There was speculation about secret agencies and genetically engineered weapons in the Cold War. Congressional hearings were held.
The causative agent, aerobic bacteria of the genus Legionella, was found in the air conditioning system of the hotel. It also occurs in ice machines, pools and fountains, water cooling systems, humidifiers and windscreen washers in cars.