Legionella bacteria occur naturally in freshwater environments like lakes, streams, and soil. In these natural settings, the bacteria exist in low concentrations that don’t typically cause disease. The problem begins when Legionella enters man-made water systems, where warm temperatures, stagnant water, and protective biofilms allow the bacteria to multiply to dangerous levels.
Natural Freshwater Is the Starting Point
Legionella species are found in rivers, lakes, streams, and moist soil worldwide. In these environments, the bacteria survive at low levels by living inside single-celled organisms called amoebae. Legionella can parasitize more than 20 species of amoebae, three species of ciliated protozoa, and even a species of slime mold. These tiny organisms serve as natural hosts, giving the bacteria a protected space to feed and reproduce. This relationship has existed long before modern plumbing, and in the wild, the bacterial concentrations stay far too low to make people sick.
The bacteria enter building water systems through the municipal water supply, which draws from these natural freshwater sources. Standard water treatment reduces Legionella levels but doesn’t always eliminate them completely. Once inside a building’s pipes, the bacteria can find conditions far more favorable for growth than anything in nature.
Man-Made Water Systems Are Where It Thrives
Legionella grows best between 77°F and 113°F (25°C to 45°C), a temperature range that overlaps perfectly with many building water systems. Cooling towers, hot tubs, decorative fountains, showerheads, and hot water distribution systems all create environments where the bacteria can flourish. Hotel water systems are a well-documented source of community-acquired Legionnaires’ disease, likely because intermittent guest use means water sits stagnant in pipes between occupancies. The bacteria have also been found in private apartment hot water systems, spas, and swimming pools.
Hospitals present especially high risk. Their large, complex plumbing networks contain areas of low flow that promote stagnation and biofilm formation. Water temperatures maintained for patient comfort often fall right in Legionella’s preferred growth range. Showers, ice machines, room humidifiers, decorative fountains, and even respiratory therapy equipment have all been linked to hospital outbreaks. In one notable case in Quebec City, even a distilled water system became colonized. The vulnerability of hospitalized patients, many of whom are immunosuppressed or connected to invasive devices, makes these exposures particularly dangerous.
How Biofilms Protect the Bacteria
One of the reasons Legionella is so persistent in plumbing systems is its ability to embed itself in biofilms, the slimy layers of microorganisms that coat the interior surfaces of pipes and tanks. Inside a biofilm, Legionella is extremely resistant to disinfectants and biocides. Multiple Legionnaires’ disease outbreaks have been traced directly to biofilm-contaminated systems.
The relationship with amoebae matters here too. When Legionella invades an amoeba, it creates a specialized compartment inside the cell that shields it from the host’s normal defenses. The bacteria replicate inside this protective bubble, then burst out in large numbers. This process actually enhances the bacteria’s ability to produce the sticky substances that help it form new biofilm. Without amoebae present, Legionella can persist in biofilms but doesn’t actively multiply. It essentially goes dormant, waiting. With amoebae, it becomes a self-sustaining cycle of growth and spread.
This dual protection, biofilm on the outside and amoebae on the inside, is why simply adding chlorine or other disinfectants to a contaminated system often isn’t enough to eliminate the bacteria.
How It Reaches Your Lungs
Legionella doesn’t spread person to person. People get sick by breathing in tiny water droplets (aerosols) that contain the bacteria, or occasionally by accidentally inhaling water into the lungs while drinking. Any device that breaks water into a fine mist can serve as the delivery mechanism: cooling towers, showers, hot tubs, decorative fountains, and humidifiers are the most common culprits.
The mortality rate for Legionnaires’ disease, the severe pneumonia caused by the bacteria, ranges from 5% to 15% depending on the patient’s age and overall health.
Stagnant Water and Dead Legs
Plumbing design plays a major role in whether Legionella gains a foothold. “Dead legs” are sections of pipe with limited or no water flow, and “dead ends” are capped-off, abandoned pipe segments. Both create pockets of stagnant water where temperatures drift into the bacteria’s growth range and disinfectant residuals decay to nothing. A systematic review found that 22 out of 24 studies showed a positive association between stagnation zones and increased Legionella colonization.
It’s not just permanent stagnation that causes problems. Intermittent use, like a guest bathroom that only runs water a few times a month, or an office building left partially vacant, creates temporary stagnation that degrades water quality in similar ways. Current building safety guidelines recognize this risk but historically focused mainly on permanent dead ends. The broader issue of temporal stagnation from irregular use is gaining more attention, particularly after many buildings sat idle during pandemic shutdowns.
Temperature Controls That Matter
Temperature is the single most practical tool for controlling Legionella. The CDC recommends storing hot water above 140°F (60°C) and keeping circulating hot water above 120°F (49°C). Cold water should stay below 77°F (25°C), though the bacteria can grow at temperatures as low as 68°F (20°C).
Many residential water heaters ship with a default setting around 120°F, which is right at the boundary. Setting your water heater to 140°F kills Legionella in the tank, but water at that temperature can scald skin in seconds. The solution is to install thermostatic mixing valves close to faucets and showerheads. These blend the dangerously hot water with cold water right at the point of use, so what reaches your hands is safe while what circulates through the pipes is hot enough to suppress bacterial growth.
Beyond temperature, regular maintenance helps. Flushing infrequently used fixtures at least weekly prevents stagnation. Cleaning showerheads, aerators, and hose attachments removes the biofilm where bacteria hide. If your home has any capped-off pipe sections from old renovations, having a plumber remove them eliminates a stagnation risk you may not have known existed.
Building Water Management Standards
For commercial and institutional buildings, ASHRAE Standard 188 establishes minimum requirements for Legionella risk management. The standard applies to anyone involved in the design, construction, operation, or maintenance of centralized building water systems. Its 2021 edition replaced permissive, advisory language with enforceable requirements, making it easier for local codes and regulations to adopt. The standard calls for water management programs that identify where conditions favor Legionella growth, set control measures at those points, and monitor the system over time. Cooling towers, hot water systems, decorative fountains, and any equipment that aerosolizes water all fall within its scope.