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. Individual cases of legionellosis are not preventable. However, they are often investigated to confirm the diagnosis and rule out an outbreak.