Legionella pneumophila is one of many species of Legionella, but only half cause disease in people. The organisms are intracellular pathogens, so cell-mediated immunity plays the most important role in host defense. Legionella continues to cause minor epidemics throughout the world, and legionellosis is a worldwide problem. Cases appear in sporadic, endemic, and epidemic fashions. Epidemics usually result from exposure to aerosolized bacteria in the hospital or in the workplace. It accounts for one to three percent of community-acquired pneumonias and 13 percent of pneumonias acquired in the hospital.
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