The What, Who, Why and How of Legionnaires’ Disease

About Exposure to legionella bacteria

What is Legionnaires’ Disease?

Why could I catch Legionnaires' Disease?

Who is most at risk to get Legionnaires’ Disease?

The How of Legionnaires’ Disease

Legionnaires’ disease usually develops 2 to 10 days after exposure to the Legionella bacteria.
Early symptoms are much like the flu, and after a short time (often a day or two) more severe pneumonia-like symptoms may appear. Symptoms include:

  • Chills
  • Cough, difficulty breathing, shortness of breath
  • Chest pain
  • High fever
  • Muscle and joint aches
  • Headache
  • Fatigue, confusion
  • Loss of appetite


When these symptoms are present, your doctor or healthcare provider will likely perform some tests to confirm how to treat your illness.  These can include:

  • Blood tests
  • Urine test
  • Chest x-ray or CT scan
  • Bronchoscopy


That sounds like a lot of different illnesses, including the flu and pneumonia – how do I KNOW if it’s Legionnaires’ disease?
If your doctor or healthcare provider diagnoses you with pneumonia, they can use a test that checks your urine for signs of Legionella bacteria..

Proper maintenance of water systems is essential. This includes regularly cleaning and disinfecting water tanks and pipes to prevent bacterial growth. Setting water heater temperatures to at least 140°F (60°C) effectively kills the bacteria. It’s vital to avoid water stagnation, especially in pipes, as this can become a breeding ground for legionella bacteria. Regularly disinfecting faucets and showerheads, where the bacteria can thrive in scale and rust, is crucial. For respiratory devices, always use sterile or distilled water. By focusing on water quality and maintenance, the risk of Legionnaires’ disease is significantly reduced.

The World Health Organization says that every year more than 3.4 million people die as a result of water related diseases, making it the leading cause of disease and death around the world.

Although the U.S. has one of the safest drinking water systems in the world, there are an estimated 4-32 million cases of acute gastrointestinal illness (AGI) per year from public drinking water systems.


According to the CDC, about 8,000 cases of Legionnaires’ disease are now reported each year in the United States. In general, the number of cases reported to the CDC has been on the rise over the past decade. This may reflect a true increase in the frequency of disease due to a number of factors including an older U.S. population, more at-risk individuals and aging plumbing infrastructure. It may also, in part, be a result of increased use of diagnostic testing.


Peer-reviewed studies indicate that drinking water systems are the primary source of Legionella bacteria. In addition, (+) The U.S. Centers for Disease Control and Prevention (CDC), World Health Organization (WHO), U.S. Environmental Protection Agency (EPA) and the U.S. Veterans Health Administration (VHA) all list drinking water systems as primary sources of legionella and Legionnaires’ disease. According to the CDC, more than half of Legionnaires’ disease outbreaks are caused by drinking water. Exposure to Legionella bacteria most often occurs when exposed to contaminated water coming out of our faucets, shower heads, and even ice machines. Drinking water systems begin with source water and the public water treatment and distribution system which provides the supply water to our homes, schools, work places, and public buildings.


Since Legionella and other bacteria are naturally occurring in our source water, our public treatment facilities use safe levels of chemicals to eliminate them. However, bacteria also lives in the biofilm and corrosion that coats the interior walls of the pipes that carry water to our homes and directly to us. Under normal conditions, the bacteria remains in the biofilm. However, our public water systems are disrupted routinely by water main breaks, fire hydrant use, flooding, source water changes, new chemical treatments, and regular maintenance. When this happens, the biofilm where the bacteria lives is often released into the public water system where it can multiply in water storage towers and stagnate in low-flow areas and is then ultimately distributed downstream to our homes and buildings where we live.


Finding a small amount of Legionella bacteria in one of your building water system, whether potable, ornamental, recreational, or in the HVAC system, does not mean it is the source of an outbreak  – it means a more thorough investigation is needed to figure out how the bacteria got there and where else in the building and surrounding area it could still potentially be found. The water used in virtually all building water systems comes from the drinking water supply. In some systems it is refreshed regularly and is sometimes treated by water management professionals on behalf of building owners. If incoming water contains bacteria, it can enter a sub-system and in most cases the water management practices consistent with ASHRAE “Best Practices” will neutralize it. Significant disruptions to the public water system can release large volumes of contaminants into a building and consume the chemical treatments—increasing the risk of bacterial growth.

It is crucial for state and local health departments to strictly follow the recommendations by the CDC when investigating the source of any Legionnaires' disease case.

What is Legionnaires' Disease?

Legionnaires’ disease is a severe case of pneumonia – an inflammation in the lungs caused by a bacterium called legionella.

Legionnaires’ disease is caused by the waterborne Legionella bacteria, named after the 1976 outbreak involving a Philadelphia convention of the American Legion. During the event, many veterans and some attendees developed a severe, previously unrecognized form of pneumonia that became known as Legionnaires’ disease. 

Up to 18,000 people are hospitalized with Legionnaires’ disease annually in the U.S. with outbreaks most common between June and October.

About 1 in 10 people who get sick from being infected by Legionella will die.  Awareness and early treatment are vital to help people fully recover from this disease.

Why Could I Catch Legionnaires' Disease?

There are two main ways you can get Legionnaires’ disease:

  1. Breathe in small droplets of water in the air that contain the bacteria
  2. Breathing in (aspirating) drinking water containing legionella

Contamination of water droplets in the air can be caused by water sprays, jets of water or contaminated water sources.  Shower heads and sink faucets can harbor the legionella bacteria. Legionella bacteria reach people when a water supply becomes contaminated.  What are the most common locations for outbreaks?

  • Hospitals
  • Hotels
  • Large buildings

Once the bacteria enters a water system, it can multiply and be distributed throughout the building, coming into contact with people through air conditioners, fountains, mist machines, humidifiers, cooling towers, showers, hot tubs and sinks.

Who is most at risk to get Legionnaires Disease?

The people at higher risk include smokers, those with asthma or COPD, chronic lung disease, and those with a suppressed immune system. 

People over the age of 50 are also at higher risk as are people with cancer or those with an underlying illness (diabetes, kidney failure, or liver failure).