The Legionellales are an order of proteobacteria. Like all proteobacteria, they are gram-negative. The Legionellales comprise two families:
- Legionellaceae
- and
- Coxiellaceae
in which major human pathogens are found. For example, Q fever is caused by Coxiella burnetii and Legionella pneumophila causes Legionnaires' disease and Pontiac fever.