What to know about legionnaires’ disease

Legionnaires’ disease is a rare and serious form of pneumonia (lung infection) caused by Legionella bacteria. It received the name in 1976, when hundreds of people became sick and 34 died after attending an American Legion convention in Philadelphia.

Legionnaires’ disease isn’t contagious, meaning it can’t be passed from person to person. Instead, it usually spreads when someone inhales mist or vapor (tiny droplets of water in the air) that contain the bacteria. Less commonly, it spreads when a person drinks contaminated water and some of it is aspirated, meaning it gets into the lungs. Someone with problems swallowing may be more prone to aspiration.

Large, complex water systems—such as those used by hotels, hospitals, and big apartment buildings—are particularly susceptible to disease spread. In places where the water system becomes contaminated by Legionella, showers and central air conditioning—even hot tubs and decorative water fountains—could expose you to the bacteria.

Outbreaks of Legionnaires’ disease

Though the illness remains relatively uncommon, the incidence of Legionnaires’ disease in the United States has risen steadily for decades. It peaked in 2018, declined during the COVID pandemic, and began climbing again in 2022.

Each year, about 6,000 diagnoses are reported, according to the Centers for Disease Control and Prevention (CDC). The actual number of infections is likely much higher, however, as Legionnaires’ is often confused with other forms of pneumonia. Some research suggests up to 18,000 people are hospitalized annually with the disease.

Most cases of Legionnaires’ disease in the U.S. are isolated events. Though they can occur any time of year, they’re often diagnosed in summer and early fall. They tend to happen in the U.S. Midwest and Northeast but have become more common in other regions.

Outbreaks do occur from time to time. In 2015, outbreaks in New York, Illinois, and Michigan killed more than a dozen people in each state. Since 2024, there have been outbreaks of Legionnaires' disease in New Hampshire, Iowa—and again in New York City. Almost all U.S. outbreaks are caused by preventable water management issues, according to the CDC.

Apartment buildings, hotels, hospitals, and other health care facilities are frequent sites of outbreaks. A 2017 CDC report analyzing outbreaks in 20 states found that about three-quarters occurred in places like hospitals, clinics, and long-term care facilities.

Symptoms and risk factors for Legionnaires’ disease

The majority of healthy people who come into contact with Legionella don’t get sick. But some people are at higher risk for infection. These include:

• Adults ages 50 and older
• People who smoke or who formerly smoked
• People with certain chronic health conditions, including cancer, chronic obstructive pulmonary disease (COPD), diabetes, or kidney or liver failure
• People with a weakened immune system, either due to a health condition or because they’re receiving medication that affects their immunity, such as chemotherapy
• People who live in a long-term care facility, such as a nursing home
• People who have been recently hospitalized

When a person does become ill, symptoms generally start 2 to 14 days following exposure. These commonly include:

• High fever
• Cough
• Shortness of breath
• Chest pain
• Headaches
• Muscle aches
• Fatigue

Nausea, vomiting, diarrhea, and confusion may develop as the illness progresses.

Left untreated, Legionnaires’ disease can lead to serious, potentially fatal health issues. These include lung or kidney failure, or septic shock. Septic shock occurs when blood pressure drops to dangerously low levels, impairing blood flow to vital organs. It’s the final stage of sepsis, when the body has a severe reaction to an infection. The death rate from Legionnaires’ is about 10 percent overall.

Legionnaires’ disease diagnosis and treatment

To diagnose Legionnaires’ disease, a physician or other healthcare provider (HCP) will consider your:

• Symptoms
• Health status
• Risk factors
• Recent travel history
• Recent stays in a healthcare facility, especially if Legionella has been identified there

They’ll also do a urine test and/or sputum (mucus or phlegm) culture to look for signs of infection. A blood test and X-ray may be used to help confirm the diagnosis, as well.

Most cases of Legionnaires’ disease are treated with oral antibiotics. A course typically runs between one and three weeks, and many people begin feeling better within a few days. The earlier treatment begins, the lower the risk of developing more serious complications.

A person with a more severe case of illness may require a hospital stay and intravenous (IV) antibiotics, which are delivered through a needle and tubing directly into a vein.

Preventing Legionnaires’ disease

There’s no vaccine for Legionnaires’ disease or surefire way to otherwise prevent infection. However, you can take steps at home to reduce your risk—of exposure to Legionella and other germs that live in water. These include the following, recommended by the CDC.

• If showers or faucets have been out of use for a while, take time to flush them. Let cold and hot water run one after the other, each for about two minutes.
• Regularly clean and air-dry household items that need water to function: humidifiers, continuous positive airway pressure (CPAP) machines, nasal rinses, all water filters, and showerheads.
• Clean hot tubs or have them cleaned regularly.
• Flush your water heater each year or when otherwise recommended by the manufacturer.
• Don’t substitute water for windshield wiper fluid in your car.
• Get in contact with your local water utility to ensure you receive water safety alerts. Reach out to them with problems in your home, such as brown water or a drop in water pressure.

If you live in an apartment, ask your building manager about what they do to prevent bacteria from collecting in the water supply. Check whether the building has a water management program that discourages the growth of Legionella bacteria.

People at higher risk for severe disease might want to avoid public water sources like hot tubs and decorative fountains. The CDC tracks Legionnaires’ disease outbreaks on its website.

While potentially dangerous, remember that Legionnaires’ disease is relatively uncommon. If you do notice symptoms or believe you may have been exposed, reach out to an HCP right away.