Trench fever (also known as "five-day fever", "quintan fever" (Latin: febris quintana), and "urban trench fever") is a moderately serious disease transmitted by body lice. It infected armies in Flanders, France, Poland, Galicia, Italy, Salonika, Macedonia, Mesopotamia, Russia and Egypt in World War I. Three noted cases during WWI were the authors J. R. R. Tolkien, A. A. Milne, and C. S. Lewis. From 1915 to 1918 between one-fifth and one-third of all British troops reported ill had trench fever while about one-fifth of ill German and Austrian troops had the disease. The disease persists among the homeless. Outbreaks have been documented, for example, in Seattle and Baltimore in the United States among injection drug users and in Marseille, France, and Burundi.

Trench fever is also called Wolhynia fever, shin bone fever, Meuse fever, His disease, and His–Werner disease or Werner-His disease (after Wilhelm His Jr. and Heinrich Werner).

The disease is caused by the bacterium Bartonella quintana (older names: Rochalimea quintana, Rickettsia quintana), found in the stomach walls of the body louse. Bartonella quintana is closely related to Bartonella henselae, the agent of cat scratch fever and bacillary angiomatosis.