What Is Legionnaires’ Disease?

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Legionnaires’ disease is an extremely serious, potentially life-threatening type of pneumonia caused by Legionella bacteria. Legionella grows naturally in water and soil—and is generally harmless in this state—but becomes dangerous when it grows in domestic and industrial water systems, air conditioning systems, or heating systems.

It most commonly occurs in structures with complex water systems, like hotels, hospitals, and cruise ships, and spreads through hot tubs, air conditioning units, hot water tanks, plumbing systems, and fountains within these structures.

Between 10,000 and 18,000 people in the United States develop Legionnaires’ disease each year by breathing in water vapor or mist containing Legionella bacteria. Many who develop Legionnaires’ disease require treatment in the intensive care unit and can suffer from long-term effects of the condition, like chronic fatigue and neuromuscular issues.

Because Legionella can grow in any human-made water system, it’s vital to know the signs and symptoms of Legionnaires’ disease. Read on to learn more about Legionnaires’ and how you can protect you and your family from this potentially deadly disease.

Legionnaires' disease symptoms
Verywell / Brianna Gilmartin

Legionnaires' Disease Symptoms

The symptoms of Legionnaires’ disease generally develop between two and 10 days after exposure to Legionella bacteria. The earliest symptoms include chills, headache, body weakness, fatigue, and a fever of 103 degrees or higher, and can get progressively worse over the first few days.

As the condition progresses, other signs and symptoms include:

  • A severe cough that may produce mucus or blood; this may be the first sign of a lung infection
  • Pneumonia
  • Shortness of breath and chest pain
  • Gastrointestinal issues, like vomiting, diarrhea, or nausea
  • Muscle aches

Legionnaires’ disease can share symptoms with many other conditions—like the common cold or the flu, or a related condition called Pontiac Fever.

If you think you or a loved one has come into contact with Legionella bacteria from an air conditioning unit, heating system, or another human-made water system, seek medical attention immediately. Early diagnosis and treatment of Legionnaires’ disease can help shorten the recovery time and prevent serious, long-term complications down the road.


The leading cause of Legionnaires’ disease is exposure to the bacterium Legionella pneumophila through large-scale water systems. It is possible for Legionella to grow in single-home plumbing systems, but is far more frequent in large builds, where the bacteria can grow and spread more easily.

It’s most commonly believed that a person can contract Legionnaires’ disease when they inhale water droplets containing the Legionella bacteria, but there are two other methods of transmission, too:

  • Aspiration: This can occur when liquids accidentally enter the lungs—like when a person chokes while drinking water, for example. Aspirating water that contains Legionella can help the bacteria enter the lungs more easily and cause pneumonia.
  • Contact with infected soil: Although it’s rare, there have been some reported cases of Legionnaires’ disease developing from contact with infected soil.

Not every single person who encounters Legionella will develop Legionnaires’ disease. Certain risk factors and lifestyle choices can make you significantly more susceptible to contracting the disease after exposure to Legionella.

Those at a Higher Risk of Developing Legionnaires’

  • People who smoke: Smoking can severely damage the lungs and make them more susceptible to lung infections.
  • Those with chronic conditions: Lung conditions like chronic obstructive pulmonary disease (COPD) and emphysema, as well as other chronic conditions, like type 2 diabetes, can up one’s risk of developing Legionnaires’.
  • People with weakened immune systems: People with HIV or AIDS, and patients taking immune system weakening medications (like corticosteroids) may develop Legionnaires’ more easily, and have a longer recovery period.
  • Anyone age 50 or over

Legionnaires’ disease can be deadly and may have severe complications, like septic shock and respiratory failure. Anyone who thinks they may have developed Legionnaires’ disease should seek emergency medical treatment.


Once admitted to the hospital, patients can expect to undergo a series of specialized tests, including a culture sample from respiratory secretions placed on media designed specifically for Legionella bacteria. The media enables the bacteria to grow at a more rapid rate, so doctors can observe and identify.

Other common tests for Legionnaires’ disease include urinary antigen testing, which detects the bacteria in the urine; and direct fluorescent antibody (DFA) tests, during which bacteria are stained and become visible under a special fluorescent microscope. These tests are done occasionally but not commonly.

A chest X-ray will also likely be performed. Even though chest X-rays are not used to directly diagnose Legionnaires’, they are used to diagnose pneumonia, after which doctors can look for the specific bacterial or viral cause.


Legionnaires’ disease is treated with antibiotics, like azithromycin, ciprofloxacin, and levofloxacin, among others. In many cases, Legionnaires’ can be successfully treated, but even healthy people usually require a bout of hospitalization. It’s important to note that early detection and diagnosis can increase the success rate of Legionnaires’ treatment, shorten the recovery period, and help prevent serious, long-term complications.

If you (or a loved one) start to show signs and symptoms of Legionnaires’ and have been in a public swimming pool or hot tub, stayed in a hotel or cruise ship, or live in a large apartment building, seek medical care immediately.

Approximately one in 10 patients with Legionnaires’ will die from the condition.

A Word From Verywell

Legionnaires’ disease is a life-threatening condition that can be contracted from a number of sources. Fortunately, many cases—especially when detected and diagnosed early—can be successfully treated with professional medical care and antibiotics. The easiest way to protect yourself is keeping your lungs healthy by avoiding smoking and wearing a protective face mask if you’re in contact with large-scale water systems for work.

Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.

By Christina Donnelly
Christina Donnelly is a freelance writer and editor who has extensively covered health and science content. She currently works at Anthem Health as a content lead.