Legionnaires’ disease is a form of pneumonia that can happen when a person breathes in the Legionella bacteria. Most cases are due to poor water management.

In 2015, around 6,000 people received a diagnosis of Legionnaire’s disease, also known as Pontiac disease, in the United States (U.S.). However, there may be more cases that are not detected.

In 1 in 10 cases, it is fatal.

The incidence of the disease increased nearly fourfold from 2000 to 2014.

Fast facts on Legionnaire’s disease:

Here are some key points about Legionnaire’s disease. More detail is in the main article.

  • Legionnaire’s disease was first noted in 1976, and it affects around 5,000 people each year in the U.S.
  • It is a pneumonia-like illness that affects the lungs and can be fatal.
  • It happens when people breathe in water droplets that carry the Legionella bacteria.
  • Many cases are preventable, if water systems, including air conditioning, are correctly maintained.
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Legionnaire's disease was first recognized in 1976, and in 2015, 6,000 cases were confirmed in the U.S.Share on Pinterest
Legionnaire’s disease was first recognized in 1976.

Legionnaires’ disease may be hard to diagnose at first, because its signs and symptoms may be very similar to other forms of pneumonia.

Signs and symptoms usually appear between 2 and 10 days after initial infection.

They typically include:

  • a high fever, possibly over 104 degrees Fahrenheit or 40 degrees Celsius
  • chills
  • a cough
  • muscle aches
  • headaches
  • confusion
  • agitation

Some patients may experience only muscle aches and a mild headache to start with, and the other signs start to appear 1 to 2 days later.

When more severe symptoms appear, the fever will be high. The muscle pains often get worse, and the patient starts to have chills.

Often, the bacteria enter the patient’s lungs, leading to a persistent cough, shortness of breath and chest pains. The cough may be dry at first, but as the infection progresses, there will be mucus and maybe blood.

Around 1 in 3 infected patients will experience nausea, vomiting, and diarrhea. Those with gastrointestinal symptoms will usually have an extremely reduced appetite.

There may be confusion and altered mental status.


Respiratory failure can occur, in which the body does not get enough oxygen from the lungs to function properly.

Kidney failure is possible, due to an accumulation of fluids and waste in the blood.

Septic shock can occur if the infection reaches the blood. A sudden drop in blood pressure means that insufficient blood will reach the vital organs, including the brain and the kidneys.

Legionnaire’s disease mostly appears as single, isolated cases, not linked to any recognized outbreak. However, at least 20 outbreaks occur each year.

Outbreaks are more likely in the summer or early autumn, but they can happen at any time of year.

Legionella bacteria commonly exist in water. They are found in rivers and lakes, and some other water sources, generally in low numbers.

Occasionally, they may get into artificial water supply systems, for example, through the evaporative condensers that are associated with air conditioning and industrial cooling. They can appear wherever water is being artificially supplied or used.

People catch Legionnaires’ disease by inhaling small droplets of tainted water, but not from drinking it. An infected person cannot generally pass the infection to another person, although it is possible in rare cases.

Legionnaires’ disease is more likely in places with complex water supply systems, such as hospitals, hotels, and buildings that cater for large numbers of people.

The Legionella bacteria exist in rivers, lakes and other places with fresh water, but numbers are usually low because the water temperature in such places is too low for the bacteria to multiply rapidly.

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Air conditioning and water systems pose a risk for Legionnaire’s disease, if they are not fully maintained.

In artificial water systems, however, water temperatures may be higher.

Temperatures between 68 and 113 degrees Fahrenheit, or from 20 to 45 degrees Celsius, can be ideal for the bacteria to multiply rapidly and spread.

This can infect major parts of the water system.

An ideal environment also provides the right food. Impurities, such as algae, sludge, rust, and lime scale are types of food for this sort of bacteria.

Hotels, hospitals, and large buildings can provide the ideal environment, if they have an air-conditioning system that uses water for cooling.

Other types of artificial water systems that can potentially become contaminated include fountains, baths, showers, water taps, gardening sprinklers, spas, and humidifiers.

Some risk factors can make a person more susceptible to Legionnaire’s disease.

Age: It is more likely to affect people over 50 years old.

Existing health conditions: People with respiratory problems, including smokers, patients with chronic obstructive pulmonary disease (COPD) and those with lung cancer, are at higher risk.

A weakened immune system: Patients with diabetes, kidney disease, leukemia and other cancers, and who are undergoing chemotherapy are at greater risk, as their immune system is compromised.

Lifestyle factors: Heavy alcohol consumption increases the risk, and one study has suggested that using cannabis may increase susceptibility.

A report from the CDC suggests that newborns delivered in a birthing tub, or water birth, may be at risk of Legionnaire’s disease.

From 2011 to 2015, there was only one case of an infant contracting the disease before the age of 1 month. However, in the first 4 months of 2016, two infants were treated for Legionnaire’s disease, and both were born in a home birthing tub.

A physician will consider the signs and symptoms, and ask whether the patient has recently spent time in a large building, such as a hotel or hospital.

Tests that can help to detect the disease include:

  • a urine test to detect antigens, the special proteins produced by the immune system to fight the Legionella bacteria
  • blood tests
  • imaging scans to check the state of the kidneys and lungs

A lumbar puncture, or spinal tap, can reveal whether the disease has affected the brain.

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Legionella bacteria are responsible for Legionnaire’s disease. Antibiotics are used to treat it.

Treatment typically involves antibiotics, given by mouth or intravenously for between 10 and 14 days.

Adverse effects may include nausea, dizziness, headaches, loss of appetite, and chest pains.

Some patients may receive intravenous fluids to prevent dehydration.

Older patients and those with a weak immune system or another chronic condition, such as diabetes, may be hospitalized.

If the lungs are badly affected, the patient will be given oxygen.

Water systems must be properly maintained to prevent Legionnaires’ disease.

The water must be kept below 20 degrees Celsius or 68 degrees Fahrenheit, or above 60 degrees Celsius or 140 degrees Fahrenheit. Impurities should not be allowed to build up in the water.

Owners and managers of commercial buildings are urged to follow guidelines regarding water safety.

The Centers for Disease Control and Prevention (CDC) state that most cases that occur could be prevented through better water management.

They note that between 2000 and 2014, “Almost all outbreaks were caused by problems preventable with more effective water management.”

Legionnaires’ disease takes its name from the first recognized outbreak, which occurred during a convention of the American Legion in Philadelphia, in July 1976.

The bacteria spread through the air conditioning system.

The first person died on 27 July, 3 days after the convention ended. During the following week, over 130 people, mostly male, were admitted to the hospital, and 25 of them died.

They had all been to the same conference, and they had all stayed at the same hotel.

In January 1977, the cause was identified as a previously unknown bacterium, which was then named Legionella. Legionnaires’ disease is also known as legionellosis.