Legionella is a type of bacteria that lives naturally in freshwater environments but becomes dangerous when it grows in human-made water systems and is inhaled as a fine mist. There are at least 60 known species, but one in particular, Legionella pneumophila, causes the vast majority of human illness. Infection can lead to two distinct conditions: Legionnaires’ disease, a serious form of pneumonia with a fatality rate around 10%, and Pontiac fever, a milder flu-like illness that resolves on its own.
Where Legionella Grows
Legionella bacteria exist in low numbers in natural lakes, rivers, and streams, where they rarely cause problems. The danger comes when they enter building water systems that provide warm, stagnant conditions ideal for rapid multiplication. The bacteria grow best between 77°F and 113°F (25°C to 45°C), and can begin growing at temperatures as low as 68°F (20°C). Water stored above 140°F (60°C) is generally too hot for Legionella to survive.
The specific systems most likely to harbor Legionella include showerheads and sink faucets, hot tubs, decorative fountains, large complex plumbing systems, and cooling towers (the large units on building rooftops used for air conditioning). Even the windshield wiper fluid tank of a vehicle can support growth. Any system where water sits at warm temperatures for extended periods, especially with low or no disinfectant, creates a hospitable environment.
How People Get Infected
Legionella spreads when contaminated water is turned into tiny droplets or mist that people breathe in. Devices like showers, faucets, hot tub jets, and cooling towers all aerosolize water, creating particles small enough to reach deep into the lungs. This is the primary route of infection. Importantly, Legionella does not spread from person to person. You can’t catch it from someone who is sick.
Not everyone exposed to Legionella becomes ill. The people most vulnerable are adults over 50, current or former smokers, those with chronic lung disease, and people with weakened immune systems. Males over 65 are disproportionately affected. In European surveillance data from 2023, men in that age group had a notification rate of 12.7 cases per 100,000 people, compared to 5.2 for women of the same age.
How Legionella Evades the Immune System
What makes Legionella particularly dangerous is its ability to hijack the very immune cells designed to destroy it. When inhaled into the lungs, the bacteria target alveolar macrophages, the white blood cells responsible for engulfing and killing pathogens. Normally, once a macrophage swallows a bacterium, it fuses the compartment with digestive sacs called lysosomes, which break the invader down with acid and enzymes.
Legionella prevents this from happening. The bacteria inject over 300 different proteins into the host cell, blocking the digestive process and creating a protected compartment where they can replicate undisturbed. The immune system can fight back if it mounts a strong enough inflammatory response, which forces the lysosomes to fuse with the bacterial compartment and acidify it. But in people with weaker immune responses, the bacteria multiply freely inside the lungs, causing progressive pneumonia.
Legionnaires’ Disease vs. Pontiac Fever
Legionella infection produces two very different illnesses depending on the severity of the body’s response.
Legionnaires’ disease is the serious form. It presents as pneumonia with fever, cough, and shortness of breath. Many patients also experience muscle aches, headache, chest pain, nausea, diarrhea, or confusion. Hospitalization is common, and the overall fatality rate is approximately 10%. For infections acquired in healthcare settings, that rate climbs to about 25%, likely because hospital patients tend to already have compromised health.
Pontiac fever is much milder. It causes chills, fatigue, fever, headache, and muscle aches, but no pneumonia. It resolves on its own without specific treatment, hospitalization is rare, and deaths are extremely uncommon. Think of it as the body reacting to the bacteria without the bacteria successfully establishing a deep lung infection.
How Legionnaires’ Disease Is Diagnosed
Legionnaires’ disease can be difficult to distinguish from other types of pneumonia based on symptoms alone, so lab testing is essential. The most commonly used test is a urine antigen test, which detects proteins shed by Legionella pneumophila serogroup 1 (the strain responsible for most cases). It has a sensitivity of 70% to 100% and specificity of 95% to 100%, making it a strong initial screening tool. Results come back quickly, often within hours.
The limitation is that it only detects one serogroup. For broader detection, PCR testing offers 95% to 99% sensitivity with over 99% specificity and can identify other Legionella species and serogroups. Culture, growing the bacteria from respiratory samples, remains the gold standard for confirmation and is 100% specific, but its sensitivity varies widely (20% to 80%) and it takes several days to yield results.
Treatment and Recovery
Legionnaires’ disease is treated with antibiotics. The classes typically used target bacteria that live inside cells, which is critical because Legionella replicates within macrophages rather than floating freely in the bloodstream. Early treatment significantly improves outcomes. Recovery time varies depending on the patient’s overall health and how quickly treatment begins, but hospitalization of a week or more is common in moderate to severe cases. Some patients, particularly older adults, experience lingering fatigue and reduced lung function for weeks to months after the acute illness resolves.
Pontiac fever requires no specific antibiotic treatment. Symptoms typically clear within two to five days with rest and supportive care like fluids and fever reducers.
How Common It Is
Reported cases of Legionnaires’ disease have been rising steadily in recent decades, partly because of improved testing and awareness. In Europe, the notification rate reached 3.2 cases per 100,000 people in 2023, up from 2.6 the year before. Four countries (France, Germany, Italy, and Spain) accounted for 72% of all reported cases in the EU. Travel-associated cases increased by 16% in 2023 compared to the prior year, reflecting the risk posed by hotel water systems, cruise ships, and resort facilities.
These numbers almost certainly undercount the true burden. Many mild cases go undiagnosed, and the urine antigen test misses infections caused by less common Legionella species.
Preventing Legionella Growth
Prevention centers on managing building water systems so Legionella cannot establish itself. The CDC’s primary recommendation is implementing a water management program that controls the two factors most important for bacterial growth: temperature and disinfectant levels.
In practical terms, this means keeping hot water stored above 140°F (60°C) and delivered to taps above 120°F, maintaining adequate disinfectant residual throughout the plumbing system, and flushing infrequently used fixtures regularly so water doesn’t stagnate. Cooling towers require routine cleaning and biocide treatment. Buildings with large or complex water systems, such as hospitals, hotels, and long-term care facilities, face the highest risk and benefit most from formal management plans that include routine Legionella testing at key points in the system.
For homeowners, the risk is considerably lower but not zero. Keeping your water heater set to at least 120°F and running taps that go unused for extended periods (like in a vacation home or guest bathroom) are simple steps that reduce the chance of Legionella colonizing your plumbing.