Definition:

Legionnaires disease (LD), caused by Gram-negative Legionella spp., can be nosocomial, community acquired or travel related. L. pneumophila is the most common cause of LD. The source of LD is potable water systems that become colonized by the microorganism, transmitted by inhalation of contaminated aerosols and aspiration. Classically, it can cause two different forms of disease in humans: LD, with an incubation period of 2-10 days, which is a multisystem illness that involves the lungs, causing pneumonia, and can cause neurological symptoms, diarrhea and high mortality (up to 50%), and Pontiac fever, with a shorter incubation period of 1-2 days, which is an acute, self-limited, influenza-like disease that does not cause pneumonia. Patients who are at risk for infection include the immunosuppressed, especially those submitted to transplantation (mainly of solid organs), those with chronic lung disease, smokers, and the elderly.