The genus Legionella is a pathogenic group of Gram-negative bacteria that includes the species `` L. pneumophila '' , causing legionellosis ( all illnesses caused by `` Legionella '' ) including a pneumonia-type illness called Legionnaires ' disease and a mild flu-like illness called Pontiac fever .
The genus Legionella is a pathogenic group of Gram-negative bacteria . It includes the species causing a pneumonia-type illness called Legionnaires ' disease , and a mild flu-like illness called Pontiac fever .