Treatment is started as soon as Legionnaires' disease is suspected, without waiting for test results. Erythromycin or a related antibiotic are the drugs of choice. In severe cases, a second drug, rifampin, can be added. . An estimated 15,000 to 30,000 persons get Legionnaires' disease in the United States each year. An additional unknown number are infected with Legionella , have no symptoms or only a mild illness, and are never diagnosed and reported.
Cases occur singly and in outbreaks.
Although outbreaks of Legionnaires' disease receive more attention, legionellosis most often occurs as a single, isolated case. Outbreaks occur most often in the summer and early fall, but cases occur year-round. Legionellosis is not a new disease. Although the Legionella bacterium was identified only recently, cases from as far back as 1947 have been confirmed. Reported cases are increasing as health-care providers become more familiar with the disease and diagnostic tests become more widely available.