Trench warfare
The frequency of combat would increase for the units of the "elite" fighting divisions; on the Allied side, these were the British regular divisions, the Canadian Corps, the French XX Corps, and the Anzacs.

Some sectors of the front saw little activity throughout the war, making life in the trenches comparatively easy. When the I Anzac Corps first arrived in France in April 1916 after the evacuation of Gallipoli, they were sent to a relatively peaceful sector south of Armentières to "acclimatise". In contrast, some other sectors were in a perpetual state of violent activity. On the Western Front, Ypres was invariably hellish, especially for the British in the exposed, overlooked salient. However, even quiet sectors amassed daily casualties through sniper fire, artillery, disease, and poison gas. In the first six months of 1916, before the launch of the Somme Offensive, the British did not engage in any significant battles on their sector of the Western Front and yet suffered 107,776 casualties.

A sector of the front would be allocated to an army corps, usually comprising three divisions. Two divisions would occupy adjacent sections of the front, and the third would be in rest to the rear. This breakdown of duty would continue down through the army structure, so that within each front-line division, typically comprising three infantry brigades (regiments for the Germans), two brigades would occupy the front and the third would be in reserve. Within each front-line brigade, typically comprising four battalions, two battalions would occupy the front with two in reserve, and so on for companies and platoons. The lower down the structure this division of duty proceeded, the more frequently the units would rotate from front-line duty to support or reserve.

During the day, snipers and artillery observers in balloons made movement perilous, so the trenches were mostly quiet. It was during these daytime hours that the soldiers would amuse themselves with trench magazines. Because of the peril associated with daytime activities, trenches were busiest at night when the cover of darkness allowed movement of troops and supplies, the maintenance and expansion of the barbed wire and trench system, and reconnaissance of the enemy's defences. Sentries in listening posts out in no man's land would try to detect enemy patrols and working parties, or indications that an attack was being prepared.
Pioneered by the Princess Patricia's Canadian Light Infantry in February 1915, trench raids were carried out in order to capture prisoners and "booty"—letters and other documents to provide intelligence about the unit occupying the opposing trenches. As the war progressed, raiding became part of the general British policy, the intention being to maintain the fighting spirit of the troops and to deny no man's land to the Germans. As well, they were intended to compel the enemy to reinforce, which exposed their troops to artillery fire.
Such dominance was achieved at a high cost when the enemy replied with their own artillery, and a post-war British analysis concluded the benefits were probably not worth the cost. Early in the war, surprise raids would be mounted, particularly by the Canadians, but increased vigilance made achieving surprise difficult as the war progressed. By 1916, raids were carefully planned exercises in combined arms and involved close co-operation between infantry and artillery.
A raid would begin with an intense artillery bombardment designed to drive off or kill the front-trench garrison and cut the barbed wire. Then the bombardment would shift to form a "box barrage", or cordon, around a section of the front line to prevent a counter-attack intercepting the raid. However, the bombardment also had the effect of notifying the enemy of the location of the planned attack, thus allowing reinforcements to be called in from wider sectors.

Dangers: Approximately 10–15 percent of all soldiers who fought in the First World War died as a result.
While the main cause of death in the trenches came from shelling and gunfire, diseases and infections were always present, and became prevalent for all sides as the war progressed. Medical procedures, while considerably more effective than at any previous time in history, were still not very helpful; antibiotics had not yet been discovered or invented. As a result, an infection caught in a trench often went untreated and could fester until the soldier died.

Injuries: The main killer in the trenches was artillery fire; around 75 percent of known casualties. Even if a soldier was not hit directly by the artillery, shell fragments and debris had a high chance of wounding those in close proximity to the blast. Artillery use increased tremendously during the war; for example, the percentage of the French army that was  artillerymen grew from 20 per cent in 1914 to 38 percent by 1918. The second largest contributor to death was gunfire (bullets from rifles and machine-guns), which was responsible for 34 per cent of French military casualties.
Once the war entered the static phase of trench warfare, the number of lethal head wounds that troops were receiving from fragmentation increased dramatically. The French were the first to see a need for greater protection and began to introduce steel helmets in the summer of 1915. The Adrian helmet replaced the traditional French kepi and was later adopted by the Belgian, Italian and many other armies. At about the same time the British were developing their own helmets. The French design was rejected as not strong enough and too difficult to mass-produce. The design that was eventually approved by the British was the Brodie helmet. This had a wide brim to protect the wearer from falling objects, but offered less protection to the wearer's neck. When the Americans entered the war, this was the helmet they chose, though some units used the French Adrian helmet.

Disease: The predominant disease in the trenches of the Western Front was trench fever. Trench fever was a common disease spread through the faeces of body lice, which were rampant in trenches. Trench fever caused headaches, shin pain, splenomegaly, rashes and relapsing fevers – resulting in lethargy for months. First reported on the Western Front in 1915 by a British medical officer, additional cases of trench fever became increasingly common mostly in the frontline troops. In 1921, microbiologist Sir David Bruce reported that over one million Allied soldiers were infected by trench fever throughout the war. Even after the Great War had ended, disabled veterans in Britain attributed their decreasing quality of life to trench fever they had sustained during wartime.
Early in the war, gas gangrene commonly developed in major wounds, in part because the Clostridium bacteria responsible are ubiquitous in manure-fertilized soil (common in western European agriculture, such as France and Belgium), and dirt would often get into a wound (or be rammed in by shrapnel, explosion, or bullet).  In 1914, 12% of wounded British soldiers developed gas gangrene, and at least 100,000 German soldiers died directly from the infection. After rapid advances in medical procedures and practices, the incidence of gas gangrene fell to 1% by 1918.
Entrenched soldiers also carried many intestinal parasites, such as ascariasis, trichuriasis and tapeworm. These parasites were common amongst soldiers, and spread amongst them, due to the unhygienic environment created by the common trench, where there were no true sewage management procedures. This ensured that parasites (and diseases) would spread onto rations and food sources that would then be eaten by other soldiers.
Trench foot was a common environmental ailment affecting many soldiers, especially during the winter. It is one of several immersion foot syndromes. It was characterized by numbness and pain in the feet, but in bad cases could result in necrosis of the lower limbs. Trench foot was a large problem for the Allied forces, resulting in 75,000 British and 2,000 American casualties. Mandatory routine (daily or more often) foot inspections by fellow soldiers, along with systematic use of soap, foot powder, and changing socks, greatly reduced cases of trench foot. In 1918, US infantry were issued with an improved and more waterproof 'Pershing boot' in an attempt to reduce casualties from trench foot.
To the surprise of medical professionals at the time, there was no outbreak of typhus in the trenches of the Western Front, despite the cold and harsh conditions being perfect for the reproduction of body lice that transmit the disease. However, on the Eastern Front an epidemic of typhus claimed between 150,000 – 200,000 lives in Serbia. Russia also suffered a globally unprecedented typhus epidemic during the last two years of the conflict that was exacerbated by harsh winters. This outbreak resulted in approximately 2.5 million recorded deaths, 100,000 of them being Red Army soldiers.  Symptoms of typhus include a characteristic spotted rash (which was not always present), severe headache, sustained high fever of 39 °C (102 °F), cough, severe muscle pain, chills, falling blood pressure, stupor, sensitivity to light, and delirium; 10% to 60% die. Typhus is spread by body lice.

Trench rats: The trenches were inhabited by millions of rats which were often responsible for the spread of diseases. Soldiers' attempts to cull hordes of trench rats with rifle bayonets were common early in the war, but the rats reproduced faster than they could be slaughtered. However, soldiers still partook in rat hunts as a form of entertainment. Rats would feed on half-eaten or uneaten rations as well as corpses.