The intensity of World War I trench warfare meant that about 10% of the fighting soldiers were killed. This compared to 5% killed during the Boer War and 4.5% killed during World War II. For British and dominion troops serving on the Western Front, the proportion of killed was 12% while the total proportion of troops who became casualties (killed or wounded) was 56%. Considering that for every front-line infantryman there were about 3 soldiers in support (artillery, supply, medical, etc.) it was highly unlikely for a fighting soldier to survive the war without sustaining some form of injury. Indeed many soldiers were injured more than once during the course of their service.

Medical services were primitive and life-saving antibiotics undiscovered. Relatively minor injuries could prove fatal through the onset of infection and gas gangrene. The Germans recorded that 12% of leg wounds and 23% of arm wounds resulted in death, mainly through infection. The Americans recorded that 44% of casualties that developed gangrene died. Half of those who were wounded in the head died and only 1% of those wounded in the abdomen survived.

Three quarters of the wounds inflicted during the war came from shell fire. The wound resulting from a shell fragment was usually more traumatic than a gunshot wound. A shell fragment would often introduce debris making it more likely that the wound would become infected. These factors meant that a soldier was three times more likely to die from a shell wound to the chest than from a gunshot wound. The blast from shell explosions could also kill by concussion. In addition to the physical effects of shell fire there was the psychological damage. Men who had to endure a prolonged bombardment would often suffer debilitating shell shock, a condition that was not well understood at the time.

As in many other wars, World War I's greatest killer was disease. Sanitary conditions in the trenches were quite poor, and common infections included dysentery, typhus, and cholera. Many soldiers suffered from parasites and related infections. Poor hygiene also led to conditions such as trench mouth and trench foot. Another common killer was exposure, since the temperature within a trench in the winter could easily fall below zero degrees celsius.

Burial of the dead was usually a luxury that neither side could easily afford. The bodies would lie in no man's land until the front line moved, by which time the bodies were often unidentifiable. On some battlefields, such as at the Nek in Gallipoli, the bodies were not buried until after the war. On the Western Front, bodies continue to be found as fields are ploughed and building foundations dug.

At various times during the war—particularly early on—official truces were organised so that the wounded could be recovered from no man's land and the dead could be buried. Generally though, the higher commands disapproved of any slackening of the offensive for humanitarian reasons and so ordered their troops not to permit enemy stretcher bearers to operate in no man's land. However, this order was almost invariably ignored by the soldiers in the trenches, who knew that it was to the mutual benefit of the fighting men of both sides to allow the wounded to be retrieved. So, as soon as hostilities ceased, parties of stretcher bearers, marked with red cross flags, would go out to recover the wounded, sometimes swapping enemy wounded for their own. There were occasions when this unofficial cease fire was exploited to conduct a reconnaissance or to reinforce or relieve a garrison.