Clinical Overview of Legionnaires' Disease

Key points

  • Legionella can cause Legionnaires' disease and Pontiac fever, collectively referred to as legionellosis.
  • Being an older adult or having certain medical conditions puts people at increased risk for these infections.
  • It's important to report cases in a timely manner so public health officials can act quickly.
Three healthcare providers wearing scrubs and masks


Legionellosis is a bacterial disease caused by Legionella. 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 can cause Legionnaires' disease and Pontiac fever, collectively referred to as legionellosis. More rarely, these bacteria can cause extrapulmonary infections.

Risk factors

Risk factors for legionellosis include:

  • Age ≥50 years
  • Chronic lung disease (such as emphysema or COPD)
  • Immune system disorders due to disease or medication
  • Smoking (current or historical)
  • Systemic malignancy
  • Underlying illness such as diabetes, renal failure, or hepatic failure

  • Exposure to hot tubs
  • Recent care at a healthcare facility
  • Recent travel with an overnight stay outside of the home
  • Use of respiratory therapy equipment

How it spreads

Legionella can be found in natural, freshwater environments, but generally isn't 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 include:

  • Large, complex plumbing systems
  • Showerheads and sink faucets
  • Hot tubs
  • Hot water tanks and heaters
  • Decorative fountains and water features
  • Cooling towers

Cooling towers are structures that contain water and a fan as part of centralized air-cooling systems for buildings or industrial processes.

In water, Legionella grows and multiplies within amoebae and ciliated protozoa, which are small one-celled organisms. Protozoa benefit Legionella by providing:

  • Nutrients for replicating and growing
  • A shelter from adverse environmental conditions

Adverse conditions can include extreme temperatures and chemicals like chlorine.

Legionnaires' disease

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 Legionnaires' disease.

Pontiac fever

It's possible that an inflammatory response to endotoxin, rather than true infection, results in Pontiac fever.

Clinical features

Legionnaires' disease and Pontiac fever present differently in terms of clinical features, symptoms, and complications.

Keep Reading: Clinical Features


Minimizing Legionella growth in complex building water systems and devices is key to preventing infection.

Timely identification and reporting of legionellosis cases are important for allowing public health officials to act quickly. This way they can identify and stop potential clusters and outbreaks by linking new cases to previously reported ones.

Reporting cases to health departments‎

Report cases and outbreaks of Legionnaires' disease to your state, territorial, or local health department. Find more information about how to contact your health department at CDC's health department directory.
Keep Reading: Reporting Cases

Disease trends

In general, reported cases of Legionnaires' disease have been increasing since the early 2000s.

Case definitions

The Council of State and Territorial Epidemiologists released the most recent case definition for legionellosis in 2020.

Keep Reading: Case Definition