Legionnaires’ disease is caused by inhaling tiny water droplets contaminated with a bacterium called Legionella pneumophila. The bacteria grow naturally in freshwater environments but become dangerous when they colonize man-made water systems that can spray fine mist into the air. Most healthy people who encounter the bacteria never get sick, but for those who do, the infection causes a serious form of pneumonia with a fatality rate of roughly 10%.
The Bacterium Behind the Disease
Legionella pneumophila was first identified after a 1976 outbreak at an American Legion convention in Philadelphia, which gave the disease its name. The bacterium thrives in warm freshwater and has two traits that make it especially hard to eliminate from water systems.
First, Legionella survives and multiplies inside single-celled organisms called amoebae that live in the same water. These amoebae act as tiny hosts, sheltering the bacteria and allowing them to reproduce. This relationship is critical because Legionella doesn’t multiply well in clean water on its own. Second, the bacterium is relatively resistant to chlorine and heat compared to many other waterborne organisms, which lets it persist in environments like hot-water tanks that would kill less hardy bacteria.
How the Bacteria Reach Your Lungs
Legionella becomes dangerous when contaminated water is broken into a fine mist or aerosol. You get infected by breathing in those microscopic droplets, not by drinking contaminated water (though people with swallowing difficulties can aspirate water into their lungs). Person-to-person transmission does not occur.
Once inhaled, the droplets carry bacteria deep into the lungs, where they encounter immune cells called alveolar macrophages. These cells are supposed to engulf and destroy invaders, but Legionella has evolved a remarkable workaround. After being swallowed by a macrophage, the bacterium injects roughly 300 different proteins into the cell. These proteins hijack the cell’s internal machinery, preventing the normal destruction process and instead converting the space into a cozy compartment where the bacteria can replicate. Without this injection system, the bacteria cannot survive inside the cell at all. So the very immune cells meant to protect you become the primary breeding ground for the infection.
Where Legionella Grows
The bacterium grows best in water between 77°F and 113°F (25°C to 45°C), and it can begin growing at temperatures as low as 68°F (20°C). That temperature range overlaps with much of the water in buildings, especially in hot water heaters, pipes with low flow, and systems where water sits stagnant for long periods. Heating water also accelerates the loss of residual disinfectant in pipes, creating pockets where the bacteria can flourish.
The man-made water systems most commonly linked to outbreaks include:
- Cooling towers: These structures, part of centralized air-cooling systems for large buildings, use fans to blow air across heated water. They are efficient aerosol generators and have caused some of the largest, most explosive outbreaks on record.
- Potable water systems: Showerheads, faucets, and other fixtures in building plumbing can harbor Legionella for years before the problem is recognized. These outbreaks tend to be slower and less dramatic than cooling tower events but can persist much longer without remediation.
- Hot tubs and hydrotherapy pools: The bubbling and jetting action aerosolizes water efficiently. Being near a running hot tub is enough exposure.
- Decorative fountains: Especially in enclosed spaces, fountains with submerged lighting and warm ambient temperatures create favorable growing conditions for the bacteria.
Biofilm, a slimy layer of microorganisms that coats the inside of pipes and tanks, gives Legionella additional protection. Sediment, scale, and corrosion inside aging plumbing systems all promote biofilm formation, which is why older buildings and complex water systems carry higher risk.
Who Gets Sick and Why
Most healthy people exposed to Legionella fight off the bacteria without developing symptoms. The people who do get sick generally fall into specific risk categories. Current and former smokers face elevated risk because smoking damages the lungs’ natural defenses. Adults over 50 are more susceptible, and the risk rises further with age.
Several chronic health conditions also increase vulnerability:
- Chronic lung disease (including COPD and emphysema)
- Cancer
- Diabetes
- Kidney failure
- Liver failure
- Weakened immune system (from conditions or from medications like chemotherapy drugs or anti-rejection drugs after organ transplants)
People with swallowing difficulties face a different pathway of infection: they may accidentally aspirate contaminated water directly into their lungs rather than inhaling aerosolized droplets.
Legionnaires’ Disease vs. Pontiac Fever
Legionella actually causes two distinct illnesses. Legionnaires’ disease is the severe form, a full-blown pneumonia that requires hospitalization and carries an overall fatality rate of about 10%. For infections acquired in healthcare settings, where patients are already medically vulnerable, the fatality rate averages 25%.
Pontiac fever is the milder form. It causes flu-like symptoms, including fever, muscle aches, and headache, but does not involve pneumonia. It typically resolves on its own within a few days without treatment. The same bacterium causes both conditions, and researchers still don’t fully understand why one person develops severe pneumonia while another gets only a brief flu-like illness.
How Buildings Prevent Legionella Growth
Because Legionella colonizes building water systems, prevention is largely an engineering and maintenance problem. The CDC outlines a structured water management program built around a few core principles: maintaining adequate disinfectant levels throughout the system, keeping water temperatures outside the bacteria’s growth range, preventing stagnation (water that sits still in dead-end pipes or unused fixtures is a prime risk), and controlling sediment, scale, corrosion, and biofilm.
Older buildings, large facilities, hospitals, and hotels require the most attention because their plumbing systems are complex, with long pipe runs and areas of low water flow. Building managers are expected to map out the entire water system, identify points where Legionella could grow and become aerosolized, set monitoring schedules, and have corrective actions ready when something falls outside safe limits. These programs are especially critical in healthcare facilities, where the population is exactly the group most vulnerable to severe infection.