Skip directly to search Skip directly to A to Z list Skip directly to navigation Skip directly to page options Skip directly to site content

Legionellosis / Legionnaires' Disease or Pontiac Fever
1996 Case Definition

Clinical Description

Legionellosis is associated with two clinically and epidemiologically distinct illnesses: Legionnaires’ disease, which is characterized by fever, myalgia, cough, pneumonia, and Pontiac fever, a milder illness without pneumonia.

Laboratory Criteria for Diagnosis

  • Isolation of Legionella from respiratory secretions, lung tissue, pleural fluid, or other normally sterile fluids, OR
  • Demonstration of a fourfold or greater rise in the reciprocal immunofluorescence antibody (IFA) titer to greater than or equal to 128 against Legionella pneumophila serogroup 1 between paired acute- and convalescent-phase serum specimens, OR
  • Detection of L. pneumophila serogroup 1 in respiratory secretions, lung tissue, or pleural fluid by direct fluorescent antibody testing, OR
  • Demonstration of L. pneumophila serogroup 1 antigens in urine by radioimmunoassay or enzyme-linked immunosorbent assay

Case Classification


A clinically compatible case that is laboratory confirmed


The previously used category of "probable case," which was based on a single IFA titer, lacks specificity for surveillance and is no longer used.