Legionnaires' is one of those odd diseases that capture the public imagination. Perhaps there's something in the name.
That came from the first outbreak ever recorded -- in July 1976 in Philadelphia at a hotel that was hosting a meeting of the American Legion - their equivalent of the British Legion. The Legionella bacterium was identified six months later for the first time.
But it probably wasn't a new bug -- it had been around for some time - it just hadn't been identified before.
Without man's intervention, Legionella would sit around in ponds and rivers quite harmlessly to humans. But if it contaminates the ever-more complex waster systems we put in our buildings, it's a different story.
This is a bug that loves warm water - between 25 and 40 degrees Celsius. And many modern water systems -- air conditioning, cooling towers, and even showers, can contain water at that temperature. The bug thrives - especially if warm water is recycled. Then if you put it in an aerosol spray of water droplets that people can breathe in, its infectious.
It can't spread from person to person, but it's often difficult to pin down where the contamination came from. Four sites are being investigated in the Edinburgh outbreak.
Trouble is the Legionella bug could have been there two weeks ago when it started the outbreak, but might have vanished by the time the investigators got round to testing for it. There could also be more cases up to this weekend -- Legionella's incubation period can be 19 days long.
But make no mistake this is a serious infection -- even with the benefit of the most modern intensive care and intravenous antibiotics, it can still claim lives - especially among the old, heavy drinkers and smokers, who are especially susceptible. The death rate is normally between 10 and 15 per cent.
The Health Protection Agency has been logging about 350-500 cases a year over the past few years in England and Wales. Most of those are single cases -- but there's nothing like a good old-fashioned outbreak to capture the attention of us journalists.