Legionnaires' disease is a rare form of pneumonia, which is also called Legionellosis.
It was named after the first outbreak, which occurred in an American hotel, which was hosting a convention of the Pennsylvania Department of the American Legion.
It is generally contracted by inhaling mists from water sources like showers, whirlpool baths and cooling towers which have become infected with the bacteria.
The most common cause of the disease is contaminated air conditioning systems; it cannot be passed from one person to another.
Legionnaires' has an incubation period of between two to 10 days.
The first symptoms are similar to flu - headaches, muscle pain and a general feeling of being unwell.
These progress to high fever, the chills, nausea, vomiting and diarrhoea.
On the second or third day the patient usually gets a dry cough and sometimes chest pains and they may have difficulty breathing.
Some patients also suffer from confusion; disorientation; hallucination and a loss of memory. Some also get pneumonia.
Legionnaires' disease does have a high fatality rate of about 5% to 15%. But if the disease is caught early enough the prognosis is better and patients are able to be treated with antibiotics.
Those with chronic diseases such as diabetes, cancer or serious kidney problems are extremely vulnerable if they catch the disease. Smokers and alcoholics are also more vulnerable.
If you are concerned about making your hotel water supply as safe as possible, one way to do this is to run the taps in the bathroom for a few minutes with the bathroom door closed. When the steam has settled you can then use the bathroom taps. This gets rid of air bubbles in the pipes which may harbour the bug.
Hotels should regularly check the systems themselves, but sometimes rooms can be left unoccupied for some time.
Firms and buildings should also ensure that they carry out good engineering practices and regularly check the operation and maintenance of air and water handling systems.