Pathogenesis and pathology of legionellosis

Infection with members of the genus Legionella can produce a wide spectrum of disease ranging from a self-limiting febrile illness to life-threatening pneumonia. The primary site of infection in the pneumonic form of the disease appears to be the lung, but dissemination to other organs is possible. Infection results in an intense alveolitis with infiltration by large numbers of mixed inflammatory cells. The legionellae are facultative intracellular pathogens which multiply within host phagocytic cells, primarily alveolar macrophages, and disrupt the bactericidal mechanisms of these cells. The role of the polymorphonuclear leukocyte is less clearly understood. Many members of the genus produce a number of toxins which may be responsible for some of the pulmonary and extrapulmonary manifestations of disease.