Legionellosis is a bacterial disease caused by Legionella that can present as either Legionnaires’ disease or Pontiac fever.
Bacteria of the genus Legionella cause Legionnaires’ disease and Pontiac fever, and, more rarely, extrapulmonary infections, collectively referred to as legionellosis. There are at least 60 different species of Legionella, most of which are considered to be pathogenic. However, the majority of disease is caused by Legionella pneumophila, particularly serogroup 1. Legionella is transmitted via inhalation of aerosolized water containing the bacteria. Less commonly, Legionella can also be transmitted via aspiration of drinking water. Legionella is not usually transmitted from person-to-person. However, a single episode of possible person-to-person transmission of Legionnaires’ disease has been reported. 
Risk factors for legionellosis include:
- Age ≥50 years
- Smoking (current or historical)
- Chronic lung disease (such as emphysema or COPD)
- Immune system disorders due to disease or medication
- Systemic malignancy
- Underlying illness such as diabetes, renal failure, or hepatic failure
- Recent travel with an overnight stay outside of the home
- Recent care at a healthcare facility
- Exposure to hot tubs
Learn four key steps that can lead to Legionella growing in building water systems and spreading to people.
Legionella can be found in natural, freshwater environments, but generally is not present in sufficient numbers to cause disease. In human-made water systems, Legionella can grow and be transmitted to susceptible hosts via aerosolization. Human-made water systems can include
- Showerheads and sink faucets
- Cooling towers (structures that contain water and a fan as part of centralized air cooling systems for buildings or industrial processes)
- Hot tubs
- Decorative fountains and water features
- Hot water tanks and heaters
- Large, complex plumbing systems
Travel is also a risk factor for disease as hotels, resorts, and cruise ships often have large, complex water systems and aerosol-generating devices. This is also true for hospitals and long-term care facilities, which also host susceptible populations.
In water, Legionella grows and multiplies within amoebae and ciliated protozoa, which are small one-celled organisms. In addition to providing nutrients for replicating and growing Legionella, protozoa also provide a shelter that protects Legionella from adverse environmental conditions, such as extreme temperatures and chemicals like chlorine. Human immune cells called alveolar macrophages look very similar to protozoa. When in human lungs, Legionella invades and grows within alveolar macrophages, mistaking them for their natural host and causing disease.
Rates of reported cases of Legionnaires’ disease continue to rise in the United States. Nearly 10,000 cases were reported in 2018. However, because Legionnaires’ disease is likely underdiagnosed, this number may underestimate true incidence. A recent study estimated that the true number of Legionnaires’ disease cases may be 1.8–2.7 times higher than what is reported.
- Correia AM, Ferreira JS, Borges V, et al. Probable person-to-person transmission of Legionnaires’ diseaseexternal icon. N Engl J Med. 2016;374:497–8.
- Collier SA, Deng L, Adam EA, et al. Estimate of burden and direct healthcare cost of infectious waterborne disease in the United States. Emerg Infect Dis. 2021;27(1):140–9.