Legionnaires’ disease: Symptoms, transmission, and causes

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.


Legionnaire's disease was first recognized in 1976, and in 2015, 6,000 cases were confirmed in the U.S.
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


a cough

muscle aches




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.

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

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