If public health officials in Edinburgh contain the outbreak of legionnaires' disease with their "shock treatment" on 16 cooling towers, there will be relief that the episode, which has claimed at least one life, has not been worse.
Proving the exact source of an outbreak can be difficult, although authorities did pinpoint the cause at the UK's most serious outbreak, in Barrow-in-Furness in August 2002, by genetically fingerprinting the legionella bacteria to the town's arts and civic centre, Forum 28. In that case, seven people died, there were 180 confirmed cases, another 314 possible cases and 2,500 people might have been affected.
Barrow council was fined £125,000 and ordered to pay £90,000 costs for failures under the Health and Safety at Work Act.
In the Barrow outbreak, however, the mortality rate was relatively low. In another case, at Stafford hospital in 1985, there were 22 deaths out of 68 confirmed cases.
The disease takes its name from an outbreak at a convention of the American Legion veterans' organisation at the Bellevue-Stratford hotel in Philadelphia in 1976. In all, 221 people were affected and 34 died. Doctors now look for it and diagnosis can be made in a variety of ways, including taking cultures from patients' sputum, collecting cells from airways or lung tissue and blood or urine tests.
Legionnaires' is caught by breathing in small droplets of contaminated water. It is not contagious and is not known to spread directly from person to person, nor can it be contracted through drinking water. Symptoms usually begin with a mild headache and muscle pain but these might only emerge between two and 14 days after exposure to the bacteria. The symptoms then worsen and might include high fever, with a temperature of 40C (104F) or more, and increasing muscle pain and chills.
Once the bacteria infect the lungs, carriers may also experience a persistent cough, later including mucus or blood, shortness of breath and chest pains. A third of people with the disease will experience nausea, vomiting, diarrhoea or loss of appetite. About half may also experience changes to their mental state. The disease is particularly dangerous for older people or those with underlying health conditions. The earlier those who develop it are treated with antibiotics the better. An estimated 10-15% of otherwise healthy people who develop the full-blown disease are expected to die. It is three times more common in men than women (although most of those who died in Barrow were women) and mostly affects the over-50s. Smokers and heavy drinkers are more prone to developing it.
It is difficult to tell how common it is. In its milder form, symptoms are similar to those of flu. Many cases would therefore not be reported. In 2009 there were 43 deaths from legionnaires' in England and Wales. There were 345 reported cases in the two countries that year and just under half of those were thought to have developed while patients were travelling either in the UK or abroad.
The bacteria are commonly found in low numbers in rivers and lakes but the problem arises when they quickly spread in complex water supply systems under warm temperatures, including cooling towers, air conditioning, spa pools, humidifiers and indoor ornamental fountains. Large buildings such as hotels, hospitals, factories, museums and office blocks are particularly vulnerable and managers of such buildings are required to follow codes of practice. Water should be kept either cooled below 20C (68F) or above 60C (140F) and there are duties of regular inspection.