But what is the history of this terrifying infection?

The arrival of a mysterious lung disease that killed one in five sparked terror throughout the US in the summer of 1976.

The setting was a convention of the American Legion, where thousands of the mainly elderly veterans descended on the Bellevue-Stratford Hotel in Philadelphia.

Within days of the start however, veterans began falling ill in large numbers with a previously unseen form of pneumonia.

In the days that followed, 221 patients developed the illness - and 34 eventually died.

The outbreak sparked mass panic in the media and public - the Cold War was still underway, and some believed it might even be an attack by enemy forces on the veterans.

The Centers for Disease Control launched a major investigation, interviewing all the disease survivors, and carrying out microscopic autopsy.

Soil, air, and water samples were taken from the hotel and its grounds, and eventually this yielded the bacterium.

Legionella, as it was named, had been thriving in the water cooling tower of the hotel, and had been pumped into every room in fine water vapours.

UK outbreaks

It was not long before the phenomenon emerged in the UK, with cases in 1977, followed by major outbreaks in the 1980s - also linked to water cooling systems.

One, in Stafford, claimed 23 lives, and prompted a Government Committee of Inquiry.

Even the BBC was not immune - an outbreak centred on its headquarters in 1988 made 79 people ill, and killed three.

World problem

Elsewhere in the world, massive outbreaks in Murcia, Spain, and the Netherlands also claimed lives.

The emergence of water cooling towers as the chief villain in these "explosive" outbreaks led to new efforts to make them safer.

The lukewarm water circulating around them was the perfect breeding ground for the bugs, which were then discharged into the atmosphere to scatter over the nearby area.

New rules

Engineer Geoff Brundrett was among those who looked at how Legionella may be "designed out" of such equipment.

He told BBC News Online: "Basically you have to keep the water either very hot or very cold - so the bacteria cannot breed.

"You also add biocide to keep bacteria numbers down."

Guidance was issued to factory owners - but a recent survey, coupled with sporadic outbreaks of the illness, suggested that many were simply not interested in protecting the public from the disease.

Mr Brundrett said: "Perhaps a third of people surveyed said they didn't bother."

Now the climate, however, is changing.

The Health and Safety Executive now orders quarterly lab tests on the water in these systems, along with stricter instructions to keep them clean, backed up with fines or even the power to close factories in the case of repeat offenders.

And there are signs that the police are also stepping in.

A case in which a factory owner was accused of corporate manslaughter following three deaths caused by Legionella is still in the courts.