In WWI trench warfare on the Western Front, which disease was most prevalent impacting Allied troops?

The predominant disease in the trenches of the Western Front that impacted Allied soldiers was trench fever, a disease that spread through the feces of body lice. It caused head aches, shin pain, splenomegaly (enlargement of the spleen), rashes and relapsing fevers- resulting in lethargy for months.

The disease was first reported on the Western Front in 1915 by a British Medical Officer. After the war, in 1921, a microbiologist named Sir David Bruce reported that over one million Allied soldiers had been infected by trench fever throughout the war (1914-1918). Even after WWI, disabled veterans in Britain attributed their decreasing quality of life to trench fever they had sustained during wartime.

Early in the war, gas gangrene was common among entrenched soldiers, often caused by wounded soldiers coming into contact with bacteria present in the trenches. This often led to amputation of limbs. In 1914, 12% of wounded British soldiers had been infected by it. After rapid advancements in wartime medical procedures and practices, the incidence of gas gangrene fell to 1% by 1918.

Various tapeworm disease took many forms caused by the presence of intestinal parasites due to the lack of adequate hygiene in the trenches that lacked waste management procedures. The disease would spread onto rations of food the men ate.

There was no outbreak of typhus on the Western Front, however on the Eastern Front an epidemic of typhus claimed 150,000 to 200,000 lives in Serbia.