On 18 January 1977 two scientists at the Centers for Disease Control in Atlanta, USA, put an end to a mystery which had been puzzling society for months.
Dr Charles C. Shepard and Dr Joseph E McDade discovered what caused Legionnaires' disease, an outbreak of what appeared to be pneumonia which had affected 182 people the summer before, most of whom had been at an American Legion convention in Philadelphia.
They all became ill soon afterwards, with symptoms including fever, a cough, congestion and fatigue, all classic signs of pneumonia; twenty-nine of them died.
Pneumonia can have various causes, but in this case the experts were stumped. They had no idea what was responsible for the so-called Legionnaires' disease.
Then, Shephard and McDade made their discovery: the disease was not caused by any of the normal agents, but by a type of bacteria which had never been identified before. They called it Legionella pneumophila and it is believed to have been responsible for many other mysterious epidemics in the past. Scientists have also discovered that there are several different varieties of this same bacteria.
The infection isn't thought to be spread from one person to another. People catch it from bacteria in water, such as air conditioning systems and showers, which is probably why so many outbreaks have occurred in hospitals and hotels.
The largest ever outbreak of Legionnaires' disease was in Murcia, Spain, in 2001. There were more than 800 suspected cases, 449 confirmed cases and six people died. On that occasion investigations blamed the cooling towers at a local hospital. However, some organisations have also been warning tourists to Spain about the possibility of catching Legionnaires' from spas and also from the cooling systems which some restaurants and bars use on their outdoor terraces at the height of the summer.
Luckily, the discovery of what caused Legionnaires' disease also helped with its treatment, because it was found to respond to antibiotics which are not very effective for some forms of pneumonia.
Nowadays, it is usually treated quickly with medication, but if left untreated it can be fatal.