Legionnaires’ disease is an acute infection of humans, principally manifested by pneumonia, that occurs in a distinctive pattern in epidemics and is caused by bacteria of the genus Legionella. Typically the incubation period – the interval between exposure to the bacterium and the onset of illness – is 2 to 10 days, with an average of 5 to 6 days, and the attack rate – the proportion of people exposed to the bacterium who become ill – is less than 5 percent. Without specific antibiotic treatment, 15 percent or more of the cases are fatal, although the percentage of fatal cases rises sharply in immunosuppressed patients.

Legionnaires’ disease is one form of presentation of Legionella infections, which are generally referred to by the umbrella term legionellosis. Another distinctive clinicoepidemiological pattern of legionellosis is Pontiac fever. Pontiac fever affects 45 to 100 percent of those exposed and has an incubation period of 1 to 2 days. Pneumonia does not occur, and all patients recover. More than 20 species of Legionella have been identified, 10 of which are proven causes of legionellosis in humans. The most common agents of human infection are Legionella pneumophila, Legionella micdadei, Legionella bozemanii, Legionella dumoffii, and Legionella longbeachae.

Legionellae are distinguished from other bacteria in being weakly staining, gram-negative, aerobic rods that do not grow on blood agar or metabolize carbohydrates, and have large proportions of branched-chain fatty acids in their cell walls and major amounts of ubiquinones with more than 10 isoprene units on the side chain.