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Knowledge Base » Health and Safety » What is Legionnaires Disease?

What is Legionnaires Disease?

Last updated on 27th February 2023

Legionnaires’ disease, also known as legionellosis is a micro-organism known as Legionella. The HSE defines a micro-organism as “A microbiological entity, cellular or non-cellular, which is capable of replication or of transferring genetic material”. Micro-organisms are microscopic and therefore cannot be seen without a microscope. Examples of micro-organisms include viruses, protozoa, fungi and bacteria. Legionella is a type of bacteria that can cause serious ill-health and infection can be fatal.

Legionella is a bacterium (singular). It is a biological agent that can present a biohazard in the workplace. Bacteria (plural) are living single-celled micro-organisms that are very different from human cells in that they do not have a nucleus. Bacteria can come in various shapes and sizes and they can be found almost anywhere on the planet. Bacteria are extremely resilient and can withstand even the harshest of conditions.

Some bacteria are harmless and can even be beneficial. However, some bacteria, such as legionella, can be harmful by infecting humans and by causing disease. As legionella can cause disease, it is known as a pathogenic bacterium.

Legionella bacteria are gram-negative and are mainly rod-shaped. Most legionella bacteria are motile, which means they can move around by using a tail-like structure known as a flagellum. As legionella bacteria are found in water, this helps them to swim.

There are over 50 different species of legionella bacteria, but the main species that can cause disease and severe illness in humans is Legionella pneumophila.

Legionella pneumophila

Legionella pneumophila (pnuemophila means love of the lung in Latin) bacteria infect the lungs and can cause a form of pneumonia. This pneumonic form is known as Legionnaires’ disease.

As legionella pneumophila can cause serious illness, it is classified in the HSE’s the Approved List of Biological Agents. There are four hazard groups in this publication, which categorises biological agents according to how infectious they are. Hazard group 1 contains biological agents that are the least hazardous and hazard group 4 contains biological agents that are most hazardous.

Legionella pneumophila and all other legionella species are classed as hazard group 2 agents. They are in group 2, as they can cause disease in humans and are a hazard to workers, but they are unlikely to spread in the community and treatment is available.

Legionella bacteria can result in diseases collectively known as legionellosis. Legionella bacteria are commonly found in the natural environment; such as in soil and freshwater (lakes, rivers and reservoirs). Legionella bacteria numbers are normally low in these environments and therefore the risk to human health is relatively low.

Legionella bacteria can also be present in various man-made commercial, industrial and domestic water systems. Some examples of these systems are as follows:

  • Hot and cold water systems, e.g. Showers and outlet taps
  • Air conditioning systems
  • Evaporative condensing units
  • Cooling towers
  • Whirlpools, spas, hydrotherapy baths, hot tubs and Jacuzzis
  • Decorative water fountains
  • Hot-water tanks
  • Large plumbing systems
  • Humidifiers
  • Fire-fighting systems, e.g. Sprinklers

These man-made systems can often provide optimal conditions for legionella and therefore encourage the growth of the bacteria.

According to Bupa about 300 to 500 people in England and Wales get it each year and nearly half of these people caught it abroad.

Conditions for legionella growth

As you have learned, legionella bacteria are normally low in number in their natural environments. Legionella bacteria will only survive and multiply where conditions are favourable.

The following conditions will influence the survival rate and growth rate of the bacteria:

  • Temperature – Temperature is very important in controlling legionella bacteria in a water system. Legionella bacteria will typically multiply between the temperatures of 20-45°C. The bacteria will be dormant below 20°C and will be killed above 60°C.
  • Nutrients – Nutrients are also important for legionella bacteria and if systems are not regularly cleaned and disinfected, this can also encourage the multiplication of the bacteria. Legionella bacteria feed on sludge, rust, biofilm and organic matter.
  • System design and maintenance – If a water system is poorly designed and poorly maintained, then this can encourage legionella bacteria to grow. For example, water storage and poor water circulation can cause stagnant water, which can increase the risk; particularly where the temperature is optimal and nutrients are available.

The risk of infection will increase if the above are not properly controlled and if legionella bacteria enter the lungs.

Hydrotherapy pool where legionella can be found in the water system

How does legionella enter the body?

A hazardous substance, including biological agents, can enter the body via ingestion (swallowing), absorption (skin contact), injection or inhalation (breathing in). These are known as routes of entry.

Coming into contact with water, that contains legionella bacteria, is only likely to be a hazard where the water source is in a form that is airborne and therefore the water droplets can be inhaled, e.g. Water vapour, mist and steam. For example, when you turn on a hot shower, you will notice steam and water vapour and this can be inhaled into the lungs.

If water is contaminated with legionella bacteria, it can also enter the lungs by aspiration. This is where liquid, such as water, is swallowed and instead of it going down the oesophagus (food pipe) into the stomach, it accidentally goes down the trachea (windpipe) and enters a person’s lungs. This is a rare route of entry and the common route is via inhalation of water droplets contaminated with legionella bacteria.

How does legionella cause infection?

Once legionella bacteria have entered a person’s lungs, they can multiply. Remember the favourable conditions for legionella growth? The average human body (37°C) temperature is perfect for legionella bacteria to grow.

As the bacteria enter the lungs, our bodies try to fight the infection. If a person is at a higher risk, and the infection is severe, the immune system can be overwhelmed by the legionella bacteria. As the immune system responds to the infection, it causes an inflammatory response which can produce fluid in the lung air sacs (alveoli). This makes it difficult for a person to breathe and this is what causes pneumonia. Pneumonia is life-threatening if not treated.

The bacteria can also enter the bloodstream via the lung gas exchange region and this can lead to serious complications, such as sepsis.


All diseases caused by legionella bacteria are collectively known as legionellosis.

Legionellosis includes diseases such as:

  • Pontiac fever
  • Lochgoilhead fever
  • Legionnaires’ disease.

You will sometimes see the above diseases referred to as legionellosis. However, the two fevers are very different from Legionnaires’ disease. Legionnaires’ disease can cause serious illness.

It is important to note that people can be infected with legionella bacteria without it causing any symptoms or disease. Disease is likely to develop where the infection is serious and where a person is at a higher risk of contracting legionellosis. The first two fevers are less serious than Legionnaires’ disease.

Pontiac fever and Lochgoilhead fever

Legionella bacteria can cause milder illnesses such as Pontiac fever and Lochgoilhead fever.

Pontiac fever

  • A mild flu-like illness
  • Affects the upper respiratory system
  • Does not infect the lungs
  • Does not result in pneumonia (non-pneumonic form)
  • Unlikely to require treatment
  • Symptoms include -Mild fever, headache, aching muscles and fatigue. Cough is not always present
  • Symptoms show between 1 and 2 days (short incubation period)
  • Symptoms last less than a week
  • Non-fatal
  • The first case was identified in Pontiac in Michigan (USA).
Women in bed suffering with symptoms of Legionella disease

Lochgoilhead fever

  • Caused by a different species of Legionella bacteria (Legionella micdadei)
  • Pontiac fever-type illness – similar symptoms
  • Named after an outbreak in Lochgoilhead (Scotland).

These illnesses are less serious that Legionnaires’ disease and usually go by themselves without treatment.

Legionnaires’ disease

Legionnaires’ disease is a serious lung infection that is caused by inhaling legionella bacteria (particularly Legionella pnuemophila). It is the pneumonic form of legionellosis.

Legionella pneumophila was first identified from an outbreak that occurred at a convention at the Bellevue-Stratford Hotel in Philadelphia in 1976. The people who were attending the convention were part of the American Legion. After the convention, 221 attendees developed pneumonia and 34 died. After an in-depth investigation, it was found that Legionella pneumophila was the cause of the outbreak and the source was a cooling tower of the hotel’s air conditioning system. It was aptly named Legionnaires’ disease, as the attendees of the convention were part of the American Legion (Legionnaires). Unlike Pontiac and Lochgoilhead fevers, Legionnaires’ disease is a form of pneumonia. It is serious and can be fatal. Anyone can be infected with legionella bacteria; however, there are factors that can increase the risk of someone developing Legionnaires’ disease.

Legionnaires’ disease is not contagious, i.e. it cannot be passed from person to person. The disease is relatively rare and people can be exposed to legionella bacteria without developing Legionnaires’ disease.

If a person does develop Legionnaires’ disease, they will develop some of the following symptoms:

  • High fever and chills
  • Persistent cough – mucus and/or blood can be present
  • Shortness of breath
  • Headache
  • Muscle pains and aches
  • Chest pain
  • Appetite loss
  • Diarrhoea
  • Nausea and vomiting
  • Fatigue (malaise)
  • Mental confusion
  • Seizures.

You will notice that some of the symptoms of Legionnaires’ disease are similar to the flu. It is therefore important to consider legionellosis where there is a risk of legionella bacteria being present.

The incubation period is the time from when a person is exposed to a biological agent and the time it takes for symptoms to show. The incubation period for Legionnaires’ disease is typically between 2-10 days, although it can be longer in some people.

Who is at risk of legionellosis?

Anyone can be infected with legionella bacteria. Some people are at a higher risk of legionellosis and the severity of infection will depend on the following risk factors:

  • Age – As age increases, so does the risk. People over 45 are at a higher risk
  • Gender – Males are at a higher risk
  • Lifestyle – People who smoke and/or drink excessive alcohol are at a higher risk, as they are damaging their lungs, kidneys and liver
  • Existing ill-health – People with existing health conditions such as diabetes, kidney disease and lung disease are at a higher risk, as their organs are already impaired by their existing conditions
  • Weakened immunity – Anyone who has a weakened immune system, such as those with HIV/AIDS, those who are being treated with chemotherapy and those who are taking corticosteroid treatments, are at a higher risk as it will be difficult to fight infections
  • Travelling – Those who travel regularly are at a higher risk, as some countries do not have robust regulations for legionella control.

Diagnosis of Legionnaires’ disease

Legionnaires’ disease is not easy to diagnose, as symptoms are very similar to the flu and other forms of pneumonia. Therefore, if workers develop a flu-like illness, and have potentially been exposed to water systems that could contain legionella bacteria, they need to visit their doctor promptly. If legionella is suspected as the cause, treatment is usually started before confirmation. This is due to the seriousness of Legionnaires’ disease if treatment is delayed.

For a diagnosis of Legionnaires’ disease to be made, certain tests need to be carried out to determine whether legionella bacteria are present.

These tests include:

  • Urine – To test for antigens. This detects a protein from the legionella bacteria
  • Sputum (phlegm) and blood – Identifies whether legionella is the cause by growing the bacteria in a laboratory from the samples
  • Chest X-rays – This cannot confirm the presence of legionella but can identify whether someone has a lung infection and/or pneumonia.
Doctor looking at X-ray results of a person's chest who is suspected to have legionella disease

It is not just workplaces where people can be exposed to legionella bacteria and this must be considered in the diagnosis.

If left untreated, Legionnaires’ disease can result in the following:

  • Respiratory Failure – The disease can cause breathing problems where the lungs do not function properly. This means that oxygen cannot get to vital organs
  • Kidney Failure – The disease can result in the kidneys not functioning properly. This leads to fluids, toxins and waste building up
  • Sepsis – This is blood poisoning due to the bacteria infecting a person’s whole body. A person can go into septic shock and this is where a person’s blood pressure drops so low that vital organs do not get enough blood and therefore cease functioning
  • Death – Legionnaires’ disease is life-threatening. It is rare, but people do die from Legionnaires’ disease and death usually occurs as a result of pneumonia. The World Health Organization (WHO) estimates that the disease death rate ranges from 5-10%.

Outbreaks of Legionnaires’ Disease

There have been several outbreaks of Legionnaires’ disease around the world. The most notable are:

1976 American Legion Hotel Convention, Philadelphia USA – You have learned about the discovery of Legionnaires’ disease and the outbreak at the American Legion conference. 221 people developed the disease and 34 died.

1985 Stafford Hospital, England – 175 patients developed Legionnaires’ disease as a result of Stafford District Hospital’s air conditioning system (cooling tower). The outbreak resulted in the deaths of 28 patients.

1999 Bovenkarspel, Netherlands – 318 people were infected and 32 people died after attending the Westfriese Flora flower exhibition. The source was a hot tub in the flower exhibition area.

2001 Hospital Murcia, Spain – This was the world’s largest outbreak where over 800 people were infected and 6 people died. The source was the hospital cooling towers.

2002 Barrow in Furness, England – 172 people were infected and 7 died. The source was a cooling tower at an arts centre.

2005 Seven Oaks Home for the Aged, Toronto Canada –127 nursing home residents were infected and 21 died. The source was the home’s air conditioning cooling towers.

2014 Portugal – This was a widespread outbreak where there were 375 cases and 12 people died. The source was a cooling tower.

2015 Lincoln Hospital and Concourse Plaza Hotel, South Bronx New York- 113 people were infected and 12 died. The source was cooling towers from the hotel and hospital.

This is only a small sample of outbreaks over the last 50 years and there is no evidence of outbreaks halting. You will notice that most of the above outbreaks result from cooling towers. This is because the water droplets can be spread over a larger area and so more people are infected.

How common is legionellosis?

Legionnaires’ disease is notifiable in the United Kingdom. As Legionnaires’ disease is notifiable, suspected infections must be reported by local health professionals. The number of cases is then collated by Public Health England and annual reports are produced. The reports contain the number of cases of Legionnaires’ disease reported. There is also a monthly national surveillance scheme in England and Wales and a monthly report is produced. Scotland and Northern Ireland have their reporting systems and schemes.

There are approximately between 300 and 500 cases of Legionnaires’ disease, in the United Kingdom, every year. There are of course exceptions to these numbers, but trends show that it is on the increase. Most cases are related to travel and approximately a third of infections are acquired abroad. Many cases of legionellosis will go undetected and unreported due to the diseases resembling the flu.

Legionnaires’ disease in the United Kingdom is rare, but it appears to peak around the summer months. As the weather gets warmer, this creates a favourable temperature for legionella bacteria. Although it may be rare in the United Kingdom, it is still a risk in the workplace and must be properly controlled.

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About the author

Eve Johnson

Eve Johnson

Eve has worked at CPD from the start, she organises the course and blog production, as well as supporting students with any problems they may have and helping them choose the correct courses. Eve is also studying for her Business Administration Level 3 qualification. Outside of work Eve likes to buy anything with flamingos on it, catching up with friends, spending time with her family and occasionally going to the gym!

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