Open Outbreak

Legionnaires' disease

Also known as: legionellosis, Legionella, Legionella pneumophila

Legionnaires' disease is a serious lung infection caused by Legionella bacteria, most often Legionella pneumophila. These bacteria live naturally in freshwater, but illness usually happens when they grow in human-made water systems such as cooling towers, hot water systems, fountains, spas, or hospital plumbing and people breathe in tiny contaminated droplets. WHO says the incubation period is usually 2 to 10 days, though up to 16 days has been recorded in some outbreaks. The disease can range from a mild cough to rapidly fatal pneumonia. WHO reports that overall death is usually about 5% to 10%, but it can be much higher in untreated immunosuppressed patients. Prompt diagnosis and appropriate antibiotic treatment matter. Outbreak control focuses on finding and cleaning the water source, not on isolating large numbers of contacts, because spread is mainly linked to contaminated water aerosols rather than routine person-to-person transmission. In the sources used here, no licensed vaccine was identified; vaccine status is therefore listed as unknown.

Key facts

TransmissionBreathing in aerosolized water contaminated with Legionella from man-made water systems; aspiration of contaminated water can also occur. Person-to-person spread is not established as a usual route in the sources used here.
IncubationUsually 2 to 10 days; up to 16 days has been recorded in some outbreaks (WHO, 2022).
Case fatalityOverall usually 5% to 10%. WHO reports 5% to 30% with appropriate case management depending on severity, and as high as 40% to 80% in untreated immunosuppressed patients.
Reproduction numberunknown
Vaccineunknown
TreatmentPrompt diagnosis and appropriate antibiotic treatment; outcome depends partly on early case management.
ReservoirFreshwater environments and human-made water systems that allow Legionella to multiply, including plumbing and cooling systems.
Endemic regionsWorldwide where suitable water systems exist; outbreaks are reported in community and healthcare settings.

Transmission

Legionnaires' disease is usually caught from water, not from another sick person. The bacteria live in freshwater and can spread when they grow in building water systems and are released in tiny droplets that people breathe in. Sources described in the materials here include cooling systems, plumbing, and other built-water environments. This is why outbreaks are often linked to hotels, hospitals, apartment blocks, spas, fountains, or industrial cooling towers rather than households with an ill family member. WHO's Argentina outbreak report also describes follow-up of contacts without reported symptoms, which fits the idea that routine close-contact spread is not the main concern. For public health, the key question is where the contaminated water source is and who was exposed to it. The disease can also be healthcare-associated when a hospital or clinic water system is involved.

Symptoms and severity

Legionnaires' disease is a form of pneumonia, meaning infection in the lungs. WHO says symptoms often begin with fever, mild cough, loss of appetite, headache, tiredness, and a general unwell feeling. Some people also have muscle pain, diarrhoea, or confusion. In the Argentina cluster, all reported cases had bilateral pneumonia, fever, muscle pain, abdominal pain, and shortness of breath. WHO says the illness can range from a mild cough to rapidly fatal pneumonia, and that untreated disease usually worsens during the first week. The incubation period is usually 2 to 10 days, though up to 16 days has been seen in some outbreaks. Severity is not the same for everyone. Older adults, smokers, people with chronic illness, and people with weakened immune systems are at higher risk of severe disease. WHO reports that overall death is usually 5% to 10%.

Treatment and vaccines

This disease needs medical care because it can become severe quickly. WHO says the death rate depends partly on whether appropriate antibiotic treatment is used and on how severe the illness is when care begins. In the same WHO report, overall mortality is usually 5% to 10%, but it may rise much higher in untreated immunosuppressed patients. A diagnostic review in the literature also notes that treatment of cases and rapid control of outbreaks benefit from swift diagnosis together with laboratory investigation. That matters because pneumonia caused by Legionella is not always obvious at first. The main public-health response is twofold: treat the patient and find the water source so others are not exposed. In the sources used for this page, I did not find a WHO fact sheet or paper stating that a licensed vaccine is available, so vaccine status is listed as unknown rather than assumed absent.

Where it occurs

Legionnaires' disease can occur in many countries because Legionella bacteria are widespread in freshwater and can colonize human-made water systems. The built environment is the key setting in the sources reviewed here. That means risk is shaped less by climate borders than by the presence and maintenance of water systems that can support bacterial growth and spread contaminated aerosols. Community outbreaks and healthcare-associated clusters are both well described. Hospital water systems matter especially because the people exposed there may already be ill or immunocompromised. WHO outbreak notices in this session show events in Europe, Latin America, and Australia over time, which supports a broad geographic range rather than a narrow endemic zone. In practical terms, the disease should be thought of as a worldwide water-system hazard, with risk concentrated in buildings and facilities where water is warm, stagnant, poorly disinfected, or aerosolized.

Recent history

On this site, the archive count supplied for legionellosis is 10 WHO notices or major outbreak records since 1996, which suggests repeated but intermittent public-health events rather than one continuous global emergency. The WHO notices returned in this session include a large outbreak in Portugal in 2014: by 2014-11-12, 302 cases had been reported and 5 deaths were confirmed, with all cases linked to the affected area. WHO also reported a healthcare-linked cluster in Argentina in 2022: 11 cases of severe pneumonia and 4 deaths, with Legionella identified from samples. The search results also listed a 2023 event in Poland with 166 cases and 23 deaths, showing that large outbreaks still occur and can appear in different settings and countries. Taken together, the recent pattern is recurring outbreaks tied to contaminated water systems, including community and healthcare exposures.

What to watch

The main warning signs are not the same as for diseases that spread easily between people. More concerning signals would be: outbreaks in new regions or unusual settings; repeated hospital-linked clusters; evidence that a contaminated source is affecting many buildings or a city's water network; delays in identifying the environmental source; and rising severe disease in older or immunocompromised people. Another important signal would be convincing evidence of sustained human-to-human transmission, because the sources used here point mainly to aerosol exposure from water systems instead. Public-health teams also need to watch for failures of water-system control, such as cooling towers or plumbing that remain contaminated after cleaning. On the clinical side, a rise in severe pneumonia cases with negative routine tests should prompt consideration of Legionella. Vaccine supply is not currently a watch item from these sources because no vaccine status beyond unknown was established here; treatment speed and source control matter more.

Updated