Legionnaire's disease: signs, treatment & prevention

Legionnaire's disease is a type of pneumonia caused by Legionella bacteria living in warm water. It can hit fast and hard — people often need hospital care. If you know the common signs and quick steps to take, you can cut the risk of serious trouble.

What to watch for and who is at risk

Typical symptoms start 2–10 days after exposure. Watch for high fever, dry cough, shortness of breath, muscle aches, headache, and sometimes nausea, diarrhea or confusion. The mix of lung and stomach symptoms is a useful clue. Older adults, smokers, people with chronic lung disease, and anyone with a weakened immune system face higher risk of severe illness.

There’s also Pontiac fever — a milder, flu-like response to the same bacteria. It usually clears on its own. Legionnaire's disease is the serious version and needs medical attention.

Diagnosis, treatment, and quick actions

If you suspect Legionnaire's disease, get medical help without delay. Doctors will run tests such as a urinary antigen test for Legionella pneumophila (the common strain), sputum culture, or PCR on respiratory samples. Early testing helps match treatment and can speed public health responses if multiple cases appear.

Treatment is antibiotic-based. Health professionals often use a macrolide (like azithromycin) or a fluoroquinolone (like levofloxacin). Hospital care is common when breathing is difficult or oxygen is needed. Don’t try to self-treat with leftover antibiotics — see a clinician for the right choice and dose.

If you or a family member has rapid breathing trouble, low oxygen, fainting, or confusion, go to the emergency room. Tell clinicians about recent travel, hotel stays, time in hotels, hot tubs, or exposure to building cooling systems — that info helps find the source faster.

When an outbreak is suspected, local health departments investigate. They test building water systems and cooling towers and recommend cleaning and disinfection steps to stop more people getting sick.

Prevention focuses on water management. Keep hot water hot (usually above 50°C/122°F where safe), avoid stagnant water, clean and disinfect cooling towers, maintain hot tubs properly, and flush unused plumbing. Building owners should follow a written water-safety plan and work with qualified contractors for testing and remediation.

Simple personal steps help too: run taps and showers after long closures, avoid mist from poorly maintained spas, and tell your doctor about any recent stays in large buildings if you get pneumonia-like symptoms.

Legionnaire's disease is treatable when found early. Knowing the symptoms, risk factors, and where the bacteria hide gives you a real advantage. If something feels wrong, act fast — that’s the best way to protect yourself and others.