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.
Unlike Trench Foot which became better controlled later in the war, this disease continued to increase in numbers throughout WW1. The human is the only known host for this louse borne disease.
The symptoms were high fever up to 104 Degrees F., leg pain, inflamed eyes, headaches, skin rashes and lymph involvement. These symptoms easily could be confused with the symptoms of Typhoid Fever or the flu. Soldiers could recover in 5-6 days, but hospitalization was not uncommon. Several relapses were expected. Today it is treated by long term antibiotic therapy lasting several weeks and relapses are expected.
For more info about diseases in the trenches, check out the other part of my series: