ATLANTA—Instances of Legionnaires' disease—a severe, sometimes fatal pneumonia—are growing in the U.S., often because hotels long-term care facilities and hospitals haven't taken enough steps to ensure their water is clean, according to a report issued Tuesday from the U.S. Centers for Disease Control and Prevention.

"Almost all Legionnaires' disease outbreaks are preventable with improvements in water systems management," CDC Director Tom Frieden said Tuesday.

The CDC reviewed 27 building outbreaks of Legionnaires' disease from 2000 to 2014 at hotels and resorts, long-term care facilities, senior living facilities and hospitals. The CDC investigators found that workers had not used enough disinfectant, didn't change filters often enough, or didn't monitor their water pipes and storage areas properly, leading to the outbreaks.

Showering with infected water was a leading source of contamination, as well as air-conditioning, hot tubs, and, in one case, a decorative fountain, according to the report.

Dr. Frieden urged building managers across the country to set up plans to figure out where the disease might grow and take steps to reduce it.

Legionnaires' disease is caused by Legionella bacteria that can grow in unclean water pipes, air-conditioning cooling towers, hot tubs and anywhere there is dirty, stagnant water. The disease can be transmitted to humans when people inhale mist of the infected water.

While most people don't contract the disease if exposed to the bacteria, older people, smokers and people with compromised immune systems can become very ill, often requiring hospitalization. Symptoms include coughing, shortness of breath, muscle aches, high fever and headache. In about one in 10 cases, the disease is fatal, according to the CDC.

In 2014, the rate of reported Legionnaires' disease per 100,000 people was 1.62, up from 0.42 per 100,000 in 2000. Possible causes for the increase in Legionnaires' disease include the aging of the population, more people with compromised immune systems and decaying plumbing in many older buildings, Dr. Frieden said.

In the past year, about 5,000 people were diagnosed with the disease and more than 20 outbreaks were reported to the CDC. Outbreaks occur more often in warmer weather.

The CDC warning comes as U.S. health officials focus their attention on water safety following the discovery that the drinking water supply of Flint, Mich., was contaminated with harmful amounts of lead, and revelations that drinking water near current and former chemical plants in upstate New York, New England and Alabama are contaminated by a potentially harmful chemical, perfluorooctanoic acid, or PFOA, once widely used in manufacturing.

In 2014 and 2015, a Legionnaires' outbreak in Flint and nearby areas caused 12 deaths and sickened more than 90, according to the Michigan Department of Health and Human Services. About half of the Flint-area cases were traced to a Flint hospital connected to the city's lead-tainted water supply. But state officials haven't found a definitive link between troubled water system and the outbreak, a state health department spokeswoman said.

Last summer [2015], the worst outbreak of Legionnaires' ever to hit New York City erupted in the South Bronx, killing 12 people and sickening more than 120. Investigators traced the source to a contaminated cooling tower.

Cynthia Whitney, chief of the CDC's respiratory diseases branch, said Tuesday that hospitals need to be especially careful about the disease, because patients often are highly vulnerable to infection.

Disinfecting works well to reduce the bacteria, although more study is needed to figure out which disinfectants work best, she said.

"This really is an area that needs more research," Ms. Whitney said.

Write to Cameron McWhirter at cameron.mcwhirter@wsj.com

(END) Dow Jones Newswires

June 07, 2016 14:15 ET (18:15 GMT)