Legionnaires’ disease is a form of atypical pneumonia caused by any type of Legionella bacteria. Signs and symptoms include cough, shortness of breath, high fever, muscle pain, and headache.


Legionella organisms are often found in soil and freshwater. Amoebae present in fresh water are a natural reservoir for these bacteria. These organisms can enter a building’s plumbing system through fresh water sources; a building’s water supply is often the cause of a Legionella epidemic. The organisms of this disease are embedded in a biofilm that forms inside water pipes and containers. Infection is usually acquired by inhaling aerosols of contaminated water. Nosocomial infection usually involves a supply of contaminated hot water. Legionella infection is not passed from person to person, although a probable case was reported in 2016.


Legionnaires’ disease is a flu-like syndrome with acute fever, chills, malaise, myalgia, headache, or confusion. Nausea, loose stools or watery diarrhea, abdominal pain, cough, and arthralgia also occur frequently. Pneumonic manifestations can include dyspnea, pleuritic pain, and hemoptysis. Fever-related bradycardia can occur, especially in severe cases.
The differential diagnosis of Legionnaires disease includes:

  • Acute respiratory distress syndrome
  • Aspiration pneumonia
  • Bacterial pneumonia
  • Bronchitis
  • Empyema
  • Gastroenteritis
  • Heart failure
  • HIV disease
  • Immunocompromised pneumonia
  • Influenza
  • Meningitis
  • Pleural effusion
  • Prostatitis
  • Respiratory distress
  • Septic shock
  • Typical, atypical, and severe community-acquired pneumonias
  • Viral pneumonia
  • Q Fever


Legionnaires’ disease requires treatment with antibiotics (medicines that kill bacteria in the body), and most cases of this illness can be treated successfully. Healthy people usually get better after being sick with Legionnaires’ disease, but they often need care in the hospital.


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