The causative agents are Legionella from water or potting mix. The most common cause of illness is the freshwater species L. pneumophila which is found in natural aquatic environments worldwide. However, artificial water systems which provide environments conducive to the growth and dissemination of Legionella represent the most likely sources of disease. 

The bacteria live in water systems at temperatures of 20 to 50 degrees Celsius (optimal 37 degrees Celsius). Legionella can survive and grow as parasites within free-living protozoa and within biofilms which develop in water systems. They can cause infections by infecting human cells using a similar mechanism to that used to infect protozoa. 

The most common form of transmission of Legionella is inhalation of contaminated aerosols. Sources of aerosols that have been linked with transmission of Legionella include air conditioning cooling towers, hot and cold-water systems, humidifiers, and whirlpool spas. Infection can also occur by aspiration of contaminated water or ice, particularly in susceptible hospital patients, and during water births. There is no direct human-to-human transmission. 

Legionnaires’ disease is believed to occur worldwide. 

The identified incidence of Legionnaires’ disease varies widely according to the level of surveillance and reporting. Since many countries lack appropriate methods of diagnosing the infection or sufficient surveillance systems, the rate of occurrence is unknown. In Europe, Australia, and the USA there are about 10–15 cases detected per million population per year. 

Of the reported cases 75–80% are over 50 years and 60–70% are male. Other risk factors for community-acquired and travel-associated legionellosis include smoking, a history of heavy drinking, pulmonary-related illness, immuno-suppression, and chronic respiratory or renal illnesses. 

Risk factors for hospital-acquired pneumonia are recent surgery, intubation, which is the process of placing a tube in the trachea, mechanical ventilation, aspiration, presence of nasogastric tubes, and the use of respiratory therapy equipment. The most susceptible hosts are immuno-compromised patients, including organ transplant recipients and cancer patients and those receiving corticosteroid treatment. 

Delay in diagnosis and administration of appropriate antibiotic treatment, increasing age and presence of co-existing diseases are predictors of death from Legionnaires’ disease. 

Legionellosis is a generic term describing the pneumonic and non-pneumonic forms of infection with Legionella. 

The non-pneumonic form (Pontiac disease) is an acute, self-limiting influenza-like illness usually lasting 2–5 days. The incubation period is from a few and up to 48 hours. The main symptoms are fever, chills, headache, malaise, and muscle pain (myalgia). No deaths are associated with this type of infection. 

Legionnaires’ disease, the pneumonic form, has an incubation period of 2 to 10 days (but up to 16 days has been recorded in some outbreaks). Initially, symptoms are fever, loss of appetite, headache, malaise, and lethargy. Some patients may also have muscle pain, diarrhoea, and confusion. There is also usually an initial mild cough, but as many as 50% of patients can present phlegm. Blood-streaked phlegm or haemoptysis occurs in about one-third of the patients. The severity of disease ranges from a mild cough to a rapidly fatal pneumonia. Death occurs through progressive pneumonia with respiratory failure and/or shock and multi-organ failure. 

Untreated Legionnaires’ disease usually worsens during the first week. In common with other risk factors causing severe pneumonia, the most frequent complications of legionellosis are respiratory failure, shock and acute kidney and multi-organ failure. Recovery always requires antibiotic treatment, and is usually complete, after several weeks or months. In rare occasions, severe progressive pneumonia or ineffective treatment for pneumonia can result in brain sequelae. 

The death rate as a result of Legionnaires’ disease depends on: the severity of the disease, the appropriateness of initial anti-microbial treatment, the setting where legionella was acquired, and host factors (for example, the disease is usually more serious in patients with immuno-suppression). The death rate may be as high as 40–80%in untreated immuno-suppressed patients and can be reduced to 5–30% through appropriate case management and depending on the severity of the clinical signs and symptoms. Overall, the death rate is usually within the range of 5–10%.

There is no vaccine currently available for Legionnaires’ disease. 

The nonpneumonic form of infection is self-limiting and does not require medical interventions, including antibiotic treatment. Patients with Legionnaires’ disease always require antibiotic treatment following diagnosis. 

The public health threat posed by legionellosis can be addressed by implementing water safety plans by authorities responsible for building safety or water system safety. These plans must be specific to the building or water system and should result in the introduction and regular monitoring of control measures against identified risks including Legionella. Although it is not always possible to eradicate the source of infection, it is possible to reduce the risks substantially. 

Prevention of Legionnaires’ disease depends on applying control measures to minimise the growth of Legionella and dissemination of aerosols. These measures include good maintenance of devices, including regular cleaning and disinfection and applying other physical (temperature) or chemical measures (biocide) to minimize growth. Some examples are: 

• the regular maintenance, cleaning, and disinfection of cooling towers together with frequent or continuous addition of biocides. 

• installation of drift eliminators to reduce dissemination of aerosols from cooling towers. 

• maintaining an adequate level of a biocide such as chlorine in a spa pool along with a complete drain and clean of the whole system at least weekly. 

• keeping hot and cold-water systems clean and either keeping the hot water above 50 °C (which requires water leaving the heating unit to be at or above 60 °C) and the cold below 20 °C or alternatively treating them with a suitable biocide to limit growth, particularly in hospitals and other health care settings, and aged-care facilities. 

• reducing stagnation by flushing unused taps in buildings on a weekly basis.

Applying such controls will greatly reduce the risk of Legionella contamination and prevent the occurrence of sporadic cases and outbreaks. Extra precautions may be required for water and ice provided to highly susceptible patients in hospitals including those at risk of aspiration (for example, ice machines can be a source of Legionella and should not be used by highly susceptible patients). Control and prevention measures must be accompanied by proper vigilance on the part of general practitioners and community health services for the detection of cases.

If conditions are favourable, the bacteria may grow increasing the risks of Legionnaires’ disease and it is therefore important to control the risks by introducing appropriate measures outlined in Legionnaires' disease - The Control of Legionella bacteria in water systems (L8), available at this link: https://www.hse.gov.uk/pubns/books/l8.htm

Legionella bacteria are widespread in natural water systems, e.g. rivers and ponds. 

However, the conditions are rarely right for people to catch the disease from these sources. Outbreaks of the illness occur from exposure to legionella growing in purpose-built systems where water is maintained at a temperature high enough to encourage growth, e.g. cooling towers, evaporative condensers, hot and cold water systems and spa pools used in all sorts of premises (work and domestic).

People contract Legionnaires’ disease by inhaling small droplets of water (aerosols), suspended in the air, containing the bacteria. Certain conditions increase the risk from legionella if: 

the water temperature in all or some parts of the system may be between 20-45 °C, which is suitable for growth 

• it is possible for breathable water droplets to be created and dispersed e.g. aerosol created by a cooling tower, or water outlets 

• water is stored and/or re-circulated 

• there are deposits that can support bacterial growth providing a source of nutrients for the organism e.g. rust, sludge, scale, organic matter, and biofilms 

Cases of Legionnaires’ disease are often the result of infections caught in the UK, but several cases occur abroad. 

Useful advice on travel can be found from the 

European Working Group for Legionella Infections at this link: https://www.ecdc.europa.eu/en/publications-data/european-technical-guidelines-prevention-control-and-investigation-infections

The symptoms of Legionnaires' disease are like the symptoms of the flu: 

• high temperature, feverishness, and chills. 

• cough. 

• muscle pains. 

• headache; and leading on to pneumonia 

• very occasionally diarrhoea and signs of mental confusion 

Legionnaires’ disease is not known to spread from person to person. 

How is it treated?

The illness is treated with an antibiotic called erythromycin or a similar antibiotic. 

If you develop the above symptoms and you are worried that it might be Legionnaires' disease, see your general practitioner. 

It is not always easy to diagnose because it is like the flu. A urine or blood test will be helpful in deciding whether an illness is Legionnaires' disease or not. When doctors are aware that the illness is present in the local community, they have a much better chance of diagnosing it earlier. 

If you suspect that your illness is because of your work then you should report this to your manager, as well as your health and safety representative and occupational health department, if you have one. There is a legal requirement for employers to report cases of Legionnaires' disease that may be acquired at their premises to the Health and Safety Executive. 

Any water system, with the right environmental conditions, could be a source for legionella bacteria growth. There is a reasonably foreseeable legionella risk if your water system: 

• has a water temperature between 20–45 °C 
• creates and/or spreads breathable droplets, e.g. aerosol created by a cooling tower, or water outlets 
• stores and/or re-circulates water 
• likely to contain a source of nutrients for the organism to grow, e.g. rust, sludge, scale, organic matter, and biofilms 

The most common sources of legionella are in man-made water systems including: 

• cooling tower and evaporative condensers 
• hot and cold-water systems 
• spa pools 

There are also several other potential risk systems that may pose a risk to exposure to legionella, e.g. humidifiers, air washers, emergency showers, indoor ornamental fountains etc. 

If you are an employer, or someone in control of premises (e.g. landlord), you have a duty to understand and manage legionella risks. All systems require a risk assessment but not all systems will require elaborate control measures. A simple risk assessment may show that the risks are low and being properly managed to comply with the law. If such cases, your assessment may be complete and you may not need to take any further action, but it is important to review your assessment regularly in case anything changes in your system.