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NOTE: A surveillance case definition is a set of uniform criteria used to define a disease for public health surveillance. Surveillance case definitions enable public health officials to classify and count cases consistently across reporting jurisdictions. Surveillance case definitions are not intended to be used by healthcare providers for making a clinical diagnosis or determining how to meet an individual patient’s health needs.

Clinical Description

An illness with acute onset, commonly characterized by fever, cough, and pneumonia that is confirmed by chest radiograph. Encephalopathy and diarrhea may also be included.

Laboratory Criteria For Diagnosis

  • Isolation of Legionella from lung tissue, respiratory secretions, pleural fluid, blood, or other normally sterile sites, OR
  • Demonstration of a fourfold or greater rise in the reciprocal immunofluorescence (IF) antibody titer to greater than or equal to 128 against Legionella pneumophila serogroup 1, OR
  • Demonstration of L. pneumophila serogroup 1 in lung tissue, respiratory secretions, or pleural fluid by direct fluorescence antibody testing, OR
  • Demonstration of L. pneumophila serogroup 1 antigens in urine by radioimmunoassay

Case Classification


A clinically compatible illness with demonstration of a reciprocal antibody titer greater than or equal to 256 from a single convalescent-phase serum specimen


A case that is laboratory confirmed


The 1995 case definition appearing on this page was previously published in the 1990 MMWR Recommendations and Reports titled Case Definitions for Public Health Surveillance.1 Thus, the 1990 and 1995 versions of the case definition are identical.


  1. CDC. (1990). Case Definitions for Public Health Surveillance. MMWR, 39(RR-13), 1-43.

Related Case Definition(s)