It causes atypical pneumonia, especially in people with reduced immunity due to old age, cigarette smoking, AIDS, cancer chemotherapy, alcoholism, corticosteroid use, etc. It’s also seen in people exposed to the bacteria in aerosols from air conditioners, humidifiers, respiratory therapy instruments, showers, and cooling towers.

It is a Gram negative bacillus that does not stain well with the Gram stain or H and E stain, so special stains are needed, such as the Dieterle silver impregnation technique and the Gimenez stain. It is difficult to culture. Charcoal yeast extract medium with a high concentration of iron and cysteine is used for the culture of Legionella.

The disease (also called Pontiac fever) presents with mild to severe influenza-like pneumonia, with cough and scanty sputum. Hemoptysis may develop in late stages. Other features include confusion, stupor, headache, and myalgia, along with GI symptoms such as abdominal pain and diarrhea.

Hyponatremia is a characteristic finding and can help support the diagnosis when the clinical features are typical. Although it causes pneumonia, the organism is fastidious, so identification is preferably done by detecting antigens in urine. Other tests include direct fluorescent antibody staining of respiratory specimens, culture on special media, PCR, and antibody detection by indirect immunofluorescence assay.