Legionnaires' disease (also called legionellosis) is a serious type of pneumonia (lung infection) caused by Legionella bacteria. It can range from mild to life-threatening. A related but milder illness caused by the same bacteria is Pontiac fever, which is flu-like and does not involve pneumonia.

What Caused Legionnaires' Disease?

Legionella bacteria (most commonly Legionella pneumophila) cause both illnesses. These bacteria occur naturally in freshwater environments like lakes, streams, and soil at low levels, where they rarely cause problems. They become a health concern when they multiply in human-made water systems.

How Legionella Gets Into Drinking Water Systems

Legionella can enter building or home plumbing from municipal water supplies or private wells (usually at very low levels). Once inside, it grows and spreads under favorable conditions:

• Warm temperatures — Optimal growth range is about 77–113°F (25–45°C). It grows poorly or dies at temperatures below ~68–80°F (20–27°C) or above ~124–140°F (51–60°C).
  
  
• Stagnant or low-flow water — Allows bacteria to multiply.
  
  
• Biofilm — Slimy protective layer on pipe surfaces where bacteria hide and thrive.
  
  
• Sediment, scale, or organic matter — Provides nutrients.
  
  
• Insufficient disinfectant — Low levels of chlorine or other residuals (common in building systems).

Common places in homes and buildings where it grows and becomes aerosolized (turned into fine mist/droplets):

• Showerheads and sink faucets
• Hot water tanks/heaters
• Hot tubs and whirlpools
• Cooling towers (in larger buildings)
• Decorative fountains or water features
• Humidifiers or other devices that create mist

It spreads to people primarily by inhaling these contaminated water droplets or mist (most common route). A less common route is aspiration (accidentally inhaling water into the lungs while drinking or swallowing, especially in people with swallowing difficulties). It is not typically spread by drinking the water itself or from person to person.

Home air conditioners (that don’t use water) and vehicle windshield washer fluid (if using proper fluid, not plain water) are generally not risks.

How It Affects the Body

After exposure (usually inhalation), symptoms of Legionnaires’ disease typically appear 2–10 days later (sometimes up to 14 days or more). It starts like many pneumonias:

• High fever (often 104°F / 40°C or higher), chills
• Cough (dry or with mucus; sometimes bloody)
• Shortness of breath, chest pain
• Muscle aches, headache, fatigue
• Sometimes nausea, vomiting, diarrhea, or confusion

It primarily affects the lungs but can spread and cause severe complications like respiratory failure, septic shock, or acute kidney failure. About 1 in 10 people who get it die; the rate is higher (up to 1 in 4) in healthcare settings or among vulnerable people.

Pontiac fever causes milder flu-like symptoms (fever, chills, headache, muscle aches) without pneumonia and usually resolves on its own in a few days.

Who is at higher risk? Most healthy people exposed do not get sick. Higher risk includes people aged 50+, current/former smokers, those with weakened immune systems (e.g., cancer, transplant meds, HIV), chronic lung disease (COPD, emphysema), diabetes, kidney or liver disease, or other serious conditions.

Minimizing Risks in Your Own Home Water System

The key is preventing Legionella from growing by controlling temperature, stagnation, biofilm, and ensuring good maintenance. Here are practical, evidence-based steps for homes (drawn from health authorities):

• Temperature control (most important for killing bacteria): Set your water heater thermostat to at least 140°F (60°C). This helps kill Legionella. To prevent scalding burns at taps (especially risky for children, elderly, or those with reduced sensation), install thermostatic mixing valves or anti-scald devices at fixtures to deliver water at a safe ~120°F (49°C) or lower. Insulate hot water pipes to maintain temperature and save energy. You may need to pasteurize water heating units and hot water lines.
  
  
• Flush stagnant water: After any period of disuse (vacations of 3+ days, renovations, or if water was shut off), run all cold and hot water faucets and showers for several minutes (start with cold, then hot until it reaches maximum temperature). Do this weekly for any infrequently used outlets. Flush the entire system starting from the main entry point. (Note: For Private Wells and Springs: You may need to shock disinfect the source and the distribution system.  For City Water Users, the authority may need to increase disinfection residuals and flush mains and the homeowner or apartment building may need to shock disinfect and flush lines). 
  
  
• Clean fixtures regularly: Remove and clean showerheads, faucet aerators, and screens to eliminate scale and biofilm. Soak them in a vinegar solution or follow manufacturer cleaning instructions. Clean or replace filters (whole-house, refrigerator, etc.) per instructions.
  
  
• Hot water heater maintenance: Flush sediment from the tank periodically (e.g., annually or per manufacturer guidelines). Have it inspected if you notice issues. You may need to pasteurize water heating units and hot water lines.
  
  
• Other devices:
• Hot tubs/pools: Maintain proper disinfectant (chlorine or bromine) and pH levels; clean filters; change water as recommended.
  
  
• Humidifiers, CPAP/BiPAP, or medical nebulizers: Use only distilled, sterile, or boiled/cooled water as recommended by the manufacturer or doctor; clean and disinfect regularly.
  
  
• Garden hoses: Drain after use and avoid leaving them filled with water in the sun. (Make sure you have the proper backflow prevention)
  
  
• Additional tips: Avoid or minimize use of recirculating decorative fountains if you or household members are high-risk. For private wells, test water quality periodically. Consider professional plumbing assessment for complex systems or persistent issues (e.g., removing “dead legs” or low-flow areas).

High-risk individuals should be especially diligent and may discuss additional options (like point-of-use filters) with a doctor or water treatment professional. Water management programs are standard for larger buildings but the same principles apply at home.

If you develop pneumonia-like symptoms after potential exposure (e.g., recent travel, hotel stay, or home system issues), seek medical care promptly and mention possible Legionella exposure—specific antibiotics are needed, as not all pneumonia treatments work against it.

Test for Waterborne Pathogens Including Legionella

References

1. Centers for Disease Control and Prevention (CDC). How Legionella Spreads. https://www.cdc.gov/legionella/causes/index.html. Published June 9, 2025.
   (Cited for transmission, growth factors in water systems, and infection routes.)
   
   
2. Centers for Disease Control and Prevention (CDC). About Legionnaires’ Disease. https://www.cdc.gov/legionella/about/index.html. Published August 6, 2025.
   (Cited for definition, symptoms, cause, and general prevention overview.)
   
   
3. Mayo Clinic. Legionnaires’ disease - Symptoms & causes. https://www.mayoclinic.org/diseases-conditions/legionnaires-disease/symptoms-causes/syc-20351747.
   
   
4. New York State Department of Health. Protect Yourself from Legionnaires’ Disease at Home. https://www.health.ny.gov/diseases/communicable/legionellosis/legionella_at_home.htm.
   (Cited for home-specific risk reduction steps, flushing, temperature, and maintenance advice.)
   
   
5. U.S. Environmental Protection Agency (EPA). Legionella in the Indoor Environment. https://www.epa.gov/indoor-air-quality-iaq/legionella-indoor-environment. Published August 27, 2025.
   (Cited for indoor/home water system control measures, temperature recommendations, and flushing guidance.)