The number of cases of Legionnaires' disease — a serious, sometimes fatal type of pneumonia — grew by nearly four times over the past several years, and almost all those cases could have been prevented by better water management, says the Centers for Disease Control and Prevention.

In June, health officials announced that the water system at a hospital in Pittsburgh is contaminated with Legionella, the bacteria that can cause the sometimes deadly disease. Just days earlier, the state health department in Hawaii announced it was investigating two confirmed cases of the disease found in people who had stayed at a resort in Kauai.

The number of people with Legionnaires’ disease quadrupled from 2000–2014. People typically get Legionnaires' by breathing in small water droplets, often in buildings like hospitals and hotels. The CDC examined 27 building-related outbreaks and found many of them were likely linked to problems such as inadequate disinfectant levels, human error, and equipment breakdowns that led to growth of Legionella bacteria in water systems.

Here's what you need to know about the sometimes deadly fatal disease.

1. What is Legionnaires' disease?

Legionnaires' disease is an infection caused by bacteria called Legionella, named after a 1976 outbreak during an American Legion convention in Philadelphia.

The bacteria lead to pneumonia.

About 5,000 cases of Legionnaires' disease are reported each year in the United States, according to the CDC. However, the number of infections may be higher because many cases are not diagnosed or reported.

2. How does Legionnaires' disease spread?

Legionella bacteria are commonly spread through airborne water droplets. Mist or vapor contaminated with the bacteria can come from whirlpool spas, cooling towers (used as air-conditioning units in large buildings), and water used for drinking and bathing, according to the CDC.

The illness can happen any time of year but is most common during the summer and early fall. Window and automobile air conditioners do not seem to allow the bacteria to grow, and the bacteria are not spread from person to person, according to the CDC.

3. What are the symptoms of Legionnaires' disease?

Legionella bacteria trigger pneumonia, which is inflammation of the lungs. Symptoms include cough, shortness of breath, high fever, muscle aches and headaches, and typically appear two to 10 days after exposure to the bacteria. There are many other causes of pneumonia, however, so diagnosing Legionnaires' can be difficult.

Chest X-rays, along with analyses of phlegm, blood or urine, can show evidence of the bacteria, according to the CDC.

In some cases, the Legionella bacteria cause a mild infection rather than a serious one. This condition is referred to as Pontiac fever, according to the CDC. Pontiac fever usually lasts two to five days, and there is no pneumonia. The condition may cause fever, headaches, and muscle aches, but the symptoms usually go away on their own.

Most people who develop Legionnaires' disease require hospitalization. About one out of every 10 people who get sick with Legionnaires' disease will die due to complications from the illness, according to the CDC.

4. Who is most at risk of Legionnaires' disease?

Most healthy people do not get sick after being exposed to the Legionella bacteria. People are more likely to develop Legionnaires' disease if they are 50 or older, smoke or used to smoke, or have a chronic lung disease or a weakened immune system, according to the National Institutes of Health.

5. How is Legionnaires' disease treated?

Antibiotics are used to treat Legionnaires' disease, according to the Mayo Clinic.

This article was originally written for LiveScience in 2012 and has been updated to reflect more recent information. It is republished with permission here.