Legionella are bacteria that are found naturally in freshwater sources. Legionella can grow and spread in artificial and human-made water systems, such as hot tubs, cooling towers, hot water tanks, decorative fountains, and large buildings with complex water systems. When inhaled, small water droplets or mist that contain Legionella can cause infection in people, which can result in two different illnesses: Legionnaires’ disease (a serious form of pneumonia) or Pontiac fever (a mild illness). Legionnaires’ disease is severe, and often requires hospitalization, though it can often be treated with antibiotics. Symptoms of Legionnaires’ disease are similar to other types of pneumonia and include cough, shortness of breath, fever, chills, headaches, muscle aches, nausea, vomiting, diarrhea, and confusion. People over the age of 50, current and former smokers, people with chronic lung disease, and people with weakened immune systems are at increased risk for Legionnaires’ disease. Pontiac fever can include symptoms of fever and muscle aches, and is a self-limiting illness for which treatment is usually not necessary. Generally, Legionnaires’ disease and Pontiac fever are not spread from person to person. The best way to prevent legionellosis is to reduce exposure to water sources where Legionella grow.