The first recognized cases of Legionnaires' disease occurred in 1976 in Philadelphia, Pennsylvania. Among attendees of a Legionnaires' convention held at the Bellevue-Stratford Hotel, 221 attendees contracted the disease and 34 of them died.
In April 1985, 175 patients were admitted to the District or Kingsmead Stafford Hospitals with chest infection or pneumonia. A total of 28 people died. Medical diagnosis showed that Legionnaires' disease was responsible and the immediate epidemiological investigation traced the source of the infection to the air-conditioning cooling tower on the roof of Stafford District Hospital.
In March 1999, a large outbreak in the Netherlands occurred during the Westfriese Flora flower exhibition in Bovenkarspel; 318 people became ill and at least 32 people died. This was the second-deadliest outbreak since the 1976 outbreak and possibly the deadliest as several people were buried before Legionnaires' disease had been diagnosed.
The world's largest outbreak of Legionnaires' disease happened in July 2001 with patients appearing at the hospital on July 7, in Murcia, Spain. More than 800 suspected cases were recorded by the time the last case was treated on July 22; 636–696 of these cases were estimated and 449 confirmed (so, at least 16,000 people were exposed to the bacterium) and six died, a case-fatality rate around 1%.
In late September 2005, 127 residents of a nursing home in Canada became ill with L. pneumophila. Within a week, 21 of the residents had died. Culture results at first were negative, which is not unusual, as L. pneumophila is a fastidious bacterium, meaning it leaves virtually no trace of itself. The source of the outbreak was traced to the air-conditioning cooling towers on the nursing home's roof.
As of November 10th, 2014 four people have died and 180 others are said to be in critical condition following an outbreak of Legionella in Portugal. The majority of cases so far have emerged from the city of Vila Franca de Xira in the outskirts of Lisbon and are being treated in hospitals of the Greater Lisbon area. The source hasn't been found.(updating daily) Atualização a 11 de Dezembro de 2014: As of November 12 2014, 302 people have been hospitalized following an outbreak of Legionella in Portugal and 7 related deaths have been reported. All cases, so far, have emerged in three civil parishes from the municipality of Vila Franca de Xira in the northern outskirts of Lisbon, Portugal and are being treated in hospitals of the Greater Lisbon area. The source is suspected to be located in the cooling towers of the fertilizer plant Fertibéria