Legionella is a genus of pathogenic gram-negative bacteria that includes the species L. pneumophila, causing legionellosis (all illnesses caused by Legionella) including a pneumonia-type illness called Legionnaires' disease and a mild flu-like illness called Pontiac fever.

Legionella may be visualized with a silver stain or cultured in cysteine-containing media such as buffered charcoal yeast extract agar. It is common in many environments, including soil and aquatic systems, with at least 50 species and 70 serogroups identified. These bacteria, however, are not transmissible from person to person; Furthermore, most people exposed to the bacteria do not become ill. Most outbreaks are traced to poorly maintained cooling towers.

The side chains of the cell wall carry the bases responsible for the somatic antigen specificity of these organisms. The chemical composition of these side chains both with respect to components and arrangement of the different sugars determines the nature of the somatic or O antigen determinants, which are essential means of serologically classifying many gram-negative bacteria.

Legionella acquired its name after an outbreak in 1976 of a then-unknown "mystery disease" sickened 221 people, causing 34 deaths. The outbreak was first noticed among attendees at a convention of the American Legion—an association of U.S. military veterans. The convention occurred in Philadelphia during the U.S. Bicentennial year on July 21–24, 1976. This epidemic among U.S. war veterans, occurring in the same city as—and within days of the 200th anniversary of—the signing of the Declaration of Independence, was widely publicized and caused great concern in the United States. On January 18, 1977, the causative agent was identified as a previously unknown bacterium subsequently named Legionella.