What Is Legionnaires' Disease? A Complete Overview

Legionnaires' disease is a serious form of pneumonia you've probably never thought about — until it turns up in the news linked to a hotel, hospital, or gym. It's caused by a bacterium lurking in water systems, and while most healthy people brush it off without knowing they were ever exposed, for those at risk it can be life-threatening.

QUICK FACTS • ~13,000 severe U.S. cases per year • 95% of patients require hospitalization • 1 in 10 cases result in death • 2–14 days from exposure to symptoms

WHAT IS LEGIONNAIRES' DISEASE?

Legionnaires' disease is a serious type of pneumonia caused by a bacterium called Legionella. Legionella most commonly causes one of two illnesses: Legionnaires' disease, a severe pneumonia, or Pontiac fever, a milder flu-like illness with fever, muscle aches, and headaches.

The disease got its name from a 1976 outbreak at an American Legion convention in Philadelphia — the first time the bacterium was identified and linked to illness. Since then, it has become a recognized and closely tracked public health concern. Reported cases of Legionnaires' disease have been increasing since the early 2000s, with a peak in 2018, and while cases dropped during the first year of the COVID-19 pandemic, they rebounded in 2021.

Unlike most respiratory illnesses, Legionnaires' disease is not contagious person-to-person. You can't catch it from a sick coworker or family member — it comes from inhaling contaminated water droplets from the environment.

WHERE DOES THE BACTERIA COME FROM?

Legionella is found naturally in freshwater. It can contaminate hot water tanks, hot tubs, and the cooling towers of large air-conditioning systems. The bacteria thrive in warm, stagnant water — and once they colonize a man-made water system, they can spread through mist or fine droplets inhaled by anyone nearby.

Home and car air-conditioning units don't use water to cool the air, so they aren't a risk for Legionella growth. The real danger lies in large-building water infrastructure.

Common sources: hotel water systems and showers, hospital plumbing, hot tubs and spas, building cooling towers, decorative fountains, gym and fitness facilities.

SYMPTOMS TO WATCH FOR

Legionnaires' disease often starts 2 to 10 days after contact with Legionella bacteria. Early symptoms frequently include muscle aches and a high fever — sometimes 104°F (40°C) or higher. By the second or third day, respiratory symptoms emerge.

• High fever (104°F+) • Cough (may include blood) • Shortness of breath • Muscle aches • Headache • Chest pain • Confusion or disorientation • Nausea or diarrhea

Seek care immediately if you develop a high fever and respiratory symptoms after visiting a hotel, spa, hospital, or large building — especially if you're in a high-risk group. Early antibiotic treatment significantly improves outcomes.

WHO IS MOST AT RISK?

Most healthy people exposed to Legionella don't get sick. However, certain factors put people at increased risk: • Adults aged 50 and older — immune response naturally weakens with age • Current and former smokers — damaged airways make it easier for bacteria to take hold • People with chronic lung diseases such as COPD, emphysema, or asthma • Individuals with weakened immune systems — from cancer treatment, organ transplants, HIV, or certain medications • Frequent travelers who stay in hotels or large accommodation facilities with shared water systems

RECENT OUTBREAKS

Legionnaires' disease is not rare — outbreaks occur regularly across the U.S. and globally. Recent cases include: • Harlem, New York — Community outbreak (Jul–Aug 2025): 114 cases / 7 deaths • Marshalltown, Iowa — Community outbreak (Aug–Sep 2025): 71 cases / 2 deaths • Westchester County, NY — Community outbreak (Jun–Sep 2025): 37 cases / 2 deaths • Orlando, Florida — Gym-linked outbreak (Nov–Dec 2025): 14 cases • Las Vegas, Nevada — Hotel-linked (Feb–May 2025): 3 cases

Underreporting is a known issue. Because Legionnaires' disease symptoms resemble other types of pneumonia, many cases are never tested for Legionella specifically. The true number of cases is likely higher than official counts reflect.

DIAGNOSIS AND TREATMENT

Doctors typically diagnose Legionnaires' disease using a urinary antigen test — a quick, non-invasive test that detects Legionella proteins in urine. Sputum cultures can also confirm the diagnosis and identify the specific strain.

Legionnaires' disease is treatable with antibiotics. Most people who get sick need care in a hospital but make a full recovery. However, about 1 out of 10 people who get Legionnaires' disease will die from the infection. Early diagnosis is the most important factor in a good outcome.

There is currently no vaccine for Legionnaires' disease. Prevention depends on proper maintenance and disinfection of water systems in large buildings and public facilities.

Tags: Legionnaires' Disease, Infectious Disease, Pneumonia, Legionella, Respiratory Health, Water Safety, Public Health