The genus Legionella is a group of more than 50 Gram-negative bacteria. Legionella has been discovered to be a genetically diverse species with 7-11% of genes strain-specific.
Upon inhalation, the bacteria can infect alveolar macrophages, where the bacteria can replicate. This results in Legionnaires’ disease and the less severe illness Pontiac fever. Legionella transmission is via inhalation of water droplets from a contaminated source that has allowed the organism to grow and spread (e.g., cooling towers). Transmission also occurs less commonly via aspiration of drinking water from an infected source.
Prodromal symptoms are flu-like, including fever, chills, and dry cough. Advanced stages of the disease cause problems with the gastrointestinal tract and the nervous system and lead to diarrhea and nausea. Other advanced symptoms of pneumonia may also present. However, the disease is generally not a threat to most healthy individuals, and tends to lead to severe symptoms more often in immunocompromised hosts and the elderly.