The short answer is that Legionnaires’ disease is a type of pneumonia. It is caused by bacteria called legionella which also causes other types of pneumonia, including Pontiac fever and Lochagoilhead fever. Legionnaires’ disease, however, is the most serious of the three. Untreated, it can be fatal.
Legionnaires’ disease is named after an outbreak of the disease during the 1976 American Legion convention in Philadelphia. The bacteria spread through airborne water droplets which can form in the cooling towers used for air-conditioning in large buildings, as well as sources of water for drinking and bathing. Legionella does not thrive in smaller air conditioning units such as those you’d find in cars or in apartment windows. And Legionnaires’ disease is not transmitted from person to person. You can read more about Legionella bacteria in What is Legionella.
Legionella bacteria cause pneumonia, which is inflammation of the lungs. Symptoms of pneumonia, which commonly appear between two and fourteen days, include a high fever, chills, coughing, aching muscles and headaches. However, there are many other causes of pneumonia. This makes Legionnaires’ disease particularly difficult to diagnose.
According to the CDC (Centers for Disease Control and Prevention), Legionella bacteria can be detected with chest X-rays, and analyses of phlegm, blood or urine. However, less than 5 percent of people infected with Legionella go on to develop Legionnaires’ disease.
Most will only suffer from Pontiac fever, which may cause fever, headaches and muscle ache, but which does not lead to pneumonia. After two to five days, the symptoms run their course and the patient makes a complete recovery without the need for medical intervention.
People who are at risk of Legionnaires’ disease are also at risk of developing pneumonia. This includes people over 45 years of age, heavy smokers or drinkers, or people who have a weakened immune system to begin with. The good news is that because Legionnaires’ disease is caused by bacteria, it can be treated with antibiotics. You can read more about its treatment in How do you treat Legionnaires’ disease.