Legionellosis is an infection caused by Legionella pneumophila, a gram-negative rod that thrives in warm aqueous environments such as drinking-water systems, hot tubs, and air-conditioning units. Transmission occurs by inhaling contaminated, aerosolized water droplets. Legionellosis is a common nosocomial infection and outbreaks are typical. Notable risk factors include smoking, chronic lung disease, advanced age, and immunosuppressive conditions. There are two forms of legionellosis: Legionnaires' disease and Pontiac fever. Patients with Legionnaires' disease present with atypical pneumonia (shortness of breath, cough), commonly in combination with various other symptoms, including gastrointestinal (e.g., diarrhea) and neurological (e.g., confusion). Laboratory abnormalities are common, especially hyponatremia. Pontiac fever is a milder, self-limiting, flu-like illness. A urine antigen test is used to confirm infection with L. pneumophila. Fluoroquinolones are the treatment of choice for Legionnaires' disease. In the US, legionellosis is a notifiable disease and steps should be taken to eliminate contaminated sources and to prevent future outbreaks.
- Frequency: : occurs rarely in infants, almost solely in adults (of any age) and typically in outbreaks
- High-risk groups 
Epidemiological data refers to the US, unless otherwise specified.
- Legionella pneumophila (gram-negative, obligate aerobic, facultative intracellular rod) causes over 90% of Legionnaires' disease outbreaks.
- The optimal water temperature for Legionella is 25–42°C (77–108°F). 
- Pontiac fever is generally due to lesser-known types of Legionella.
Path of infection
- Inhalation of contaminated aerosols
- Cold and hot water systems; (e.g., those found in hotels, hospitals, and retirement homes)
- Whirlpools/hot tubs, swimming pools, showers
- Air-conditioning systems with contaminated condensed water
- Person-to-person transmission is uncommon.
Since transmission from person to person is uncommon, isolation is unnecessary.
Locations at particular risk of outbreak
- Nursing homes
- Confined travel accommodations (e.g., cruise ships, hotels, resorts)
- Incubation period: 2–10 days
- Fever, chills, headache
- Severe pneumonia
- Relative bradycardia (uncommon)
- Neurological features, especially confusion, agitation, and stupor
- Failure to respond to beta-lactam monotherapy
- Incubation period: 1–3 days
- Mild, self-limiting course of legionellosis without pneumonia.
- Flu-like symptoms (e.g. fever, headache, and muscle ache)
See “Diagnosis of pneumonia”.
- Respiratory secretions
- PCR: high sensitivity, high specificity 
- Serology: : A four-fold rise in antibody titer confirms legionellosis. However, the antibody titers have low specificity and sensitivity, and seroconversion can take up to 12 weeks. Therefore, more rapid tests, such as the urinary antigen test or PCR, are more often used.
The legionaries drew their iron swords, donned their silver helmets, and jumped off the ship to burn the town to coal: legionellosis, iron buffered medium, silver stain, history of cruise ship travel, charcoal yeast extract agar.
- Chest x-ray: : Diffuse reticular opacities are commonly seen (especially in atypical pneumonia).
- Bilateral or unilateral consolidative changes and/or
- may also be seen.
- If atypical pneumonia is suspected but not yet verified: see “Medical treatment of pneumonia”.
- If legionellosis is verified:
- Drug of choice: fluoroquinolones (preferably levofloxacin, alternatively moxifloxacin) for 7–10 days
- Initial parenteral treatment is recommended for all patients to avoid possibly poor gastrointestinal absorption 
- Second-line treatment: macrolides (e.g., erythromycin or azithromycin) for 3 weeks (IV at first, later orally)
- If patients are unresponsive to monotherapy, consider adding rifampin or tigecycline.