Legionnaires' disease, or legionellosis, is a lung infection (pneumonia) caused by bacteria of the species Legionella.
This organism was first discovered in 1976, when 221 delegates at an American Legion conference in Philadelphia fell mysteriously ill. Of the 221 delegates, 34 died. After the possibility of food poisoning had been ruled out, it became apparent that they had inhaled a bacterium that was later named after these first known victims.
This bacterium had caused a similar outbreak 2 years earlier in 1974 at the same hotel, when 11 delegates became ill, although this outbreak went unnoticed. Other studies that looked at samples from 81 patients who became ill with an unknown respiratory infection at a Washington, DC, psychiatric hospital in 1965 confirmed that Legionella was the culprit. It appears that Legionella had been causing disease for many years before it was discovered, and it only became a serious threat with the arrival of modern, mechanically ventilated buildings.
Most cases of Legionnaires' disease are isolated and sporadic. Although it occurs most commonly in the late summer and early fall, it can occur at any time of the year.