Trench Fever is caused by a Gm positive bacterial rod, Bartonella quintana. It was considered non life threatening. Today this is rarely fatal unless there is no treatment of the disease or endocarditis is a factor.
Trench Fever during WW1 was considered a significant disease by the military, and affected over 1,000,000 soldiers. The disease is transmitted by a bite from the human body louse, but the disease causing organism was the bacteria located in the feces of the human body louse. The body louse bites the human. This bite offers the bacteria located in the louse fecal material an opening into the tissue thus infecting that person. Later another louse bites the infected human and then it can transmit the infection to another human host.