Gram ⊝ rod, facultative intracellular, obligate aerobe.- Aerosol transmission from environmental water source habitat (eg, air conditioning systems, hot water tanks). No person-to-person transmission.- Predisposing factors: smokers age >55 with high alcohol intake, chronic lung disease, immunosuppressed patients such as renal transplant patients.

Legionnaires' disease- Severe atypical pneumonia (often unilateral and lobar)- High fever with relative bradycardia- Watery diarrhea - Headache & confusion- Common in smokers and in chronic lung diseasePontiac fever- Mild flu-like syndrome- Pneumonitis

Lab findings:- Hyponatremia- Urine antigen test- Sputum gram stains showing many neutrophils, but few or no organisms → requires silver stain. - Legionella culture: charcoal yeast extract medium with iron and cysteine.

Treatment: macrolide or quinolone