About Legionella (Legionnaires’ Disease)
Page last updated 4/14/2026.
Legionnaires’ disease is a serious lung infection caused by Legionella bacteria.
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Legionella are bacteria naturally found in freshwater environments, such as lakes and streams. The bacteria can become a health concern when they grow and spread in human-made building water systems like:
Showerheads and sink faucets
Cooling towers (structures that contain water and a fan as part of centralized air-cooling systems for buildings or industrial processes)
Decorative fountains and water features
Hot tubs
Hot water tanks and heaters
Large, complex plumbing systems
Small water droplets or mist from these sources can contain Legionella bacteria, which people can breathe in or inhale, and lead to an infection in the lungs. Severe infection with Legionella (also called “legionellosis”) is called Legionnaires’ disease. The milder form is called Pontiac fever.
The best way to prevent disease is to stop Legionella from growing and spreading.
Building owners and managers can use a water management program to reduce the risk of Legionella growing and spreading. Building water systems and devices that might grow and spread Legionella include showerheads and sink faucets, cooling towers (structures that contain water and a fan as part of centralized air-cooling systems for buildings or industrial processes), hot tubs, decorative fountains and water features, hot water tanks and heaters, and large, complex plumbing systems.
Transmission (Spread)
People can get Legionnaires’ disease when they breathe in small droplets of water in the air that have Legionella bacteria in them. Less commonly, people can catch the infection by accidentally getting water into the lungs while drinking, also known as aspiration.
Legionnaires’ disease does not spread from person to person.
Symptoms are similar to other types of pneumonia (lung infection), such as:
- Cough
- Shortness of breath
- Fever
- Muscle aches
- Headaches
Symptoms can start 2 to 14 days after contact with the bacteria.
Legionnaires’ disease can be seen on a chest x-ray. There are many types of infections that cause pneumonia. To find out if the pneumonia is caused by Legionella, there are two commonly used tests. These are a urine test and a test of phlegm from the lung.
Treatment is with antibiotics. Most people recover, but many need hospital care. In severe cases, it can lead to lung failure or death.
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Contact the Epidemiology Unit at (619) 692-8499, or send an e-mail, for more information
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