Legionnaires disease is a form of atypical pneumonia caused by any species of Legionella bacteria, quite often Legionella pneumophila. Signs and symptoms include cough, shortness of breath, high fever, muscle pains, and headaches. Nausea, vomiting, and diarrhea may also occur. This often begins 2–10 days after exposure.A legionellosis is any disease caused by Legionella, including Legionnaires disease (a pneumonia), Pontiac fever (a nonpneumonia illness), and Pittsburgh pneumonia, but Legionnaires disease is the most common, so mentions of legionellosis often refer to Legionnaires disease. The bacterium is found naturally in fresh water. It can contaminate hot water tanks, hot tubs, and cooling towers of large air conditioners. It is usually spread by breathing in mist that contains the bacteria. It can also occur when contaminated water is aspirated. It typically does not spread directly between people, and most people who are exposed do not become infected. Risk factors for infection include older age, a history of smoking, chronic lung disease, and poor immune function. Those with severe pneumonia and those with pneumonia and a recent travel history should be tested for the disease. Diagnosis is by a urinary antigen test and sputum culture.No vaccine is available. Prevention depends on good maintenance of water systems. Treatment of Legionnaires disease is with antibiotics. Recommended agents include fluoroquinolones, azithromycin, or doxycycline. Hospitalization is often required. The fatality rate is around 10% for healthy persons and 25% for those with underlying conditions.The number of cases that occur globally is not known.
About 8,000 to 18,000 cases of Legionnaires disease occur each year in the United States, according to the Bureau of Communicable Disease Control.Between 1995 and 2005, over 32,000 cases of Legionnaires disease and more than 600 outbreaks were reported to the European Working Group for Legionella Infections. The data on Legionella are limited in developing countries, and Legionella-related illnesses likely are underdiagnosed worldwide. Improvements in diagnosis and surveillance in developing countries would be expected to reveal far higher levels of morbidity and mortality than are currently recognised. Similarly, improved diagnosis of human illness related to Legionella species and serogroups other than Legionella pneumophila would improve knowledge about their incidence and spread.A 2011 study successfully used modeling to predict the likely number of cases during Legionnaires outbreaks based on symptom onset dates from past outbreaks. In this way, the eventual likely size of an outbreak can be predicted, enabling efficient and effective use of public-health resources in managing an outbreak.During the COVID-19 pandemic, some researchers and organisations raised concerns about the impact of the COVID-19 lockdowns on Legionnaires disease outbreaks. Additionally, at least two people in England died from a co-infection of Legionella and SARS-CoV-2. Outbreaks
An outbreak is defined as two or more cases where the onset of illness is closely linked in time (weeks rather than months) and space, where a suspicion  or evidence exists of a common source of infection, with or without microbiological support (i.e.