The citizens of Edinburgh are understandably worried. A lethal infection has swept through an area in the south west of the city claiming one life and infecting 20 others, 12 of whom are critically ill. More cases are expected before the outbreak peaks. It is a scenario out of the Hollywood blockbuster Contagion.

Unlike the star of that movie, however - an avian flu virus that triggered a global pandemic - Legionnaires disease does not spread from person to person. For that we should all be thankful. With a death rate of one in ten it would be a very scary prospect indeed.

Legionnaire’s disease is a very severe form of pneumonia spread via water droplets in the air. The first outbreak occurred in 1976 at a convention for members of the American Legion at a hotel in Philadelphia. In all 221 people were taken ill and 34 died. Six months later, the cause was identified as a previously unknown bacterium, later called legionella. The air conditioning in the hotel was blamed.

The 1976 outbreak remains the world’s worst. There have been two serious outbreaks in Britain - at Stafford hospital in 1985 when 175 people were infected and 28 died and at Barrow in Furness in 2002 when 172 people were infected and seven died. The cause of both incidents was traced to contaminated cooling towers, the prime suspect in the current outbreak.

Should Edinburgh residents be worried? The number of cases rose today and is expected to go on rising until the end of the week because the disease has a long incubation period of up to 14 days. The authorities say they have tackled the likely source of the outbreak, by disinfecting industrial cooling towers on four sites in the neighbourhood?

Will that be enough? Provided they have accurately identified the source - yes. But it will take up to ten days to confirm with tests, and even then we may not know for certain.

In the meantime, the best defence against the bacterium is vigilance. Legionnaires disease can be successfully treated with antibiotics provided treatment is started promptly when symptoms appear. These will be worrying times for people living in the Dalry, Gorgie and Saughton areas of the city - but staying alert for signs of flu-like illness and prompt medical treatment when they occur should see them through.