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.
Legionnaires disease is the cause of an estimated 2–9% of pneumonia cases that are acquired outside of a hospital. An estimated 8,000 to 18,000 cases a year in the United States require hospitalization. Outbreaks of disease account for a minority of cases. While it can occur any time of the year, it is more common in the summer and fall. The disease is named after the outbreak where it was first identified, at a 1976 American Legion convention in Philadelphia. Signs and symptoms
The length of time between exposure to the bacteria and the appearance of symptoms (incubation period) is generally 2–10 days, but can more rarely extend to as long as 20 days. For the general population, among those exposed, between 0.1 and 5.0% develop the disease, while among those in hospital, between 0.4 and 14% develop the disease.Those with Legionnaires disease usually have fever, chills, and a cough, which may be dry or may produce sputum. Almost all experience fever, while around half have cough with sputum, and one-third cough up blood or bloody sputum. Some also have muscle aches, headache, tiredness, loss of appetite, loss of coordination (ataxia), chest pain, or diarrhea and vomiting. Up to half of those with Legionnaires disease have gastrointestinal symptoms, and almost half have neurological symptoms, including confusion and impaired cognition.