Legionella organisms are small, pleomorphic, gram-negative bacilli that are approximately 0.5 μm wide and 3 μm long. Their ultrastructural features are typical of gram-negative bacilli and include a cell wall with trilaminar cytoplasmic and outer membranes. L. pneumophila has a single polar flagellum; other species, except L. oakridgenesis, have polar or subpolar flagella.

The best medium for isolating Legionella is buffered charcoal-yeast extract agar supplemented with alpha-ketoglutarate. Cysteine and amino acids are required for growth. Growth occurs optimally at 35°C in 5% carbon dioxide. Colonies of L. pneumophila and other species are 1 to 2 mm in diameter, have a ground-glass appearance, are gray to gray-white in color, and exhibit a sticky consistency when lifted with a loop. The addition of dyes to the agar, fluorescence techniques, and modification of L-cysteine requirements may be used in the laboratory to differentiate species.

Of the 42 species of Legionella, fewer than one-half have been associated with human disease. L. pneumophila is the most pathogenic and accounts for 90% of legionellosis cases. L. pneumophila has 14 serogroups, each of which has been associated with human infection. Serogroup 1 accounts for 80% of reported infections.

Legionella from clinical specimens are not visible by Gram stain. Special stains, such as the Gimenez and Dieterle silver impregnation, are necessary for visualization. When obtained from tissue specimens, L. micdadei is acid-fast by a modified Ziehl-Neelsen stain or Kinyoun carbol-fuchsin technique. Other species are not acid-fast.

Legionella spreads by the airborne route, and Legionnaires disease is transmitted by inhalation, particularly of water vapor-containing aerosolized bacteria. Outbreaks have been linked to the evaporative condensers of air cooling systems, which amplify spread of the bacteria, and to soil-associated sites of excavation, whirlpool spas, showers, respiratory therapy equipment, and the ultrasonic humidifier of a grocery store mist machine.

The incidence of Legionnaires disease peaks in the late summer and early fall and has a male gender dominance. Person-to-person spread has not been documented. The incidence peaks in the sixth decade of life, and infection is an uncommon occurrence in the first two decades. Risk factors for infection in adults include smoking and alcoholism. Major predisposing features for all ages include organ transplantation, immunosuppression, malignancy, and renal disease. Underlying respiratory disease may be a risk factor in childhood.

Seroconversion to L. pneumophila or a closely related or cross-reacting organism in association with mild or inapparent clinical infection appears to be a common occurrence among young children. In one longitudinal 5-year study, more than one-half of the participants younger than 4 years of age at enrollment developed a fourfold or greater rise in titer that was not associated with acute illness. In another investigation, the frequency of reciprocal antibody titers of at least 256 was 25% among children ages 2 to 9 years of age, some of whom were tested during episodes of respiratory tract infection. The infrequency with which L. pneumophila can be implicated as a cause of acute pneumonia in physiologically normal children was illustrated by a study of 110 children who ranged in age from 1 week to 17 years and were hospitalized with pneumonia. Only two cases of Legionnaires disease—one confirmed and one possible—were identified.