Legionellosis

Legionellosis refers to any disease caused by Legionella species, including Legionnaires' disease as an atypical pneumonia and Pontiac fever as a related upper respiratory illness ^[1]. The most frequent agent is Legionella pneumophila, found naturally in fresh water and artificial systems, with L. longbeachae associated with potting mixes ^[1][2].

Affected individuals commonly develop cough, dyspnea, high fever, myalgia, and headache, with possible nausea, vomiting, and diarrhea, typically beginning two to ten days after exposure ^[1]. The illness ranges from a mild influenza-like syndrome to severe pneumonia requiring intensive care in up to one-third of Legionnaires' disease cases ^[1][4]. Extrapulmonary manifestations and nonspecific laboratory findings may accompany severe lung injury ^[4].

Legionnaires' disease is believed to occur globally, with detected incidence varying by surveillance quality; many nations lack adequate diagnosis or reporting systems so true occurrence is unknown ^[2]. In Europe, Australia, and the USA, roughly 10–15 confirmed cases per million population are seen yearly, with 75–80% of reports in those over 50 and 60–70% male ^[2]. Overall incidence has increased worldwide due to more patients with immunosuppression and better diagnostics, and fatality ranges from 4% to 40%, reaching 28% in hospital-acquired cases and up to 50% in nosocomial infections with delayed treatment ^[2][5][4]. Occupational reviews document 805 worker cases over 1949–2015 with 4.1% mortality, mostly from L. pneumophila ^[5].

Legionella naturally inhabits fresh water and can contaminate hot tubs, cooling towers, and building water tanks ^[1]. Humans are typically exposed by inhaling aerosols containing the bacteria or by aspirating contaminated water, with no direct person-to-person spread and most exposed people remaining uninfected ^[1]. Short exposures and distances up to several kilometers from a source have produced illness, depending on aerosol dissemination and source concentration ^[2].

No vaccine exists, so prevention relies on proper maintenance of water systems to limit bacterial growth and aerosol release ^[1].

1. 1 Wikipedia contributors. Legionnaires' disease - Wikipedia [Internet]. Wikipedia. cited 23 Oct 2026. Available from: https://en.wikipedia.org/wiki/Legionnaires'_disease
2. 2 World Health Organization. Legionellosis [Internet]. cited 3 Sept 2026. Available from: https://www.who.int/news-room/fact-sheets/detail/legionellosis
3. 3 Viasus D et al. Legionnaires' Disease: Update on Diagnosis and Treatment. Infect Dis Ther. 2022 Jun. PMID: 35505000. doi: 10.1007/s40121-022-00635-7. PubMed: https://pubmed.ncbi.nlm.nih.gov/35505000/
4. 4 Rello J et al. Severe Legionnaires' disease. Ann Intensive Care. 2024 Apr 2. PMID: 38565811. doi: 10.1186/s13613-024-01252-y. PubMed: https://pubmed.ncbi.nlm.nih.gov/38565811/
5. 5 Principe L et al. Legionellosis in the occupational setting. Environ Res. 2017 Jan. PMID: 27717486. doi: 10.1016/j.envres.2016.09.018. PubMed: https://pubmed.ncbi.nlm.nih.gov/27717486/