Since then, more than 39 species and 61 serogroups of legionella bacteria have been recognized. Legionella pneumophila , the first legionella bacteria species identified, accounts for approximately 90 percent of infections. Legionella can cause Pontiac Fever, an often undiagnosed and generally mild and self-limited respiratory illness. It can also cause Legionnaire's Disease, a potentially severe bacterial pneumonia that is accompanied by fever, fatigue and cough. The CDC estimates between 10,000 and 20,000 cases occur each year in the United States.
Nosocomial cases have a higher mortality rate than community-acquired cases.