Trench warfare
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. Many soldiers were more afraid of rats than other horrors found in the trenches.

Psychological impact: Nervous and mental breakdowns amongst soldiers were common, due to unrelenting shellfire and the claustrophobic trench environment. Men who suffered such intense breakdowns were often rendered completely immobile, and were often seen cowering low in the trenches, unable even to perform instinctive human responses such as running away or fighting back. This condition came to be known as "shell shock", "war neurosis" or "battle hypnosis". Although trenches provided cover from shelling and small-arms fire, they also amplified the psychological effects of shell shock, as there was no way to escape a trench if shellfire was coming. If a soldier became too debilitated from shell shock, they were evacuated from the trench and hospitalized if possible. In some cases, shell shocked soldiers were executed for "cowardice" by their commanders as they became a liability. This was often done by a  firing squad composed of their fellow soldiers – often from the same unit. Only years later would it be understood that such men were suffering from shell shock. During the war, 306 British soldiers were officially executed by their own side.

Circumvention: Throughout World War I, the major combatants slowly developed different ways of breaking the stalemate of trench warfare; the Germans focused more on new tactics while the British and French focused on tanks.

Infiltration tactics: As far back as the 18th century, Prussian military doctrine (Vernichtungsgedanke) stressed manoeuvre and force concentration to achieve a decisive battle. The German military searched for ways to apply this in the face of trench warfare. Experiments with new tactics by Willy Rohr, a Prussian captain serving in the Vosges mountains in 1915, got the attention of the Minister of War. These tactics carried Prussian military doctrine down to smallest units — specially trained troops manoeuvred and massed to assault positions they chose on their own. During the next two years the German army tried to establish special stormtrooper detachments in all its units by sending selected men to Rohr and have those men then train their comrades in their original units.
Similar tactics were developed independently in other countries, such as French Army captain André Laffargue in 1915 and Russian general Aleksei Brusilov in 1916, but these failed to be adopted as any military doctrine.
The German stormtrooper methods involved men rushing forward in small groups using whatever cover was available and laying down covering fire for other groups in the same unit as they moved forward. The new tactics, intended to achieve surprise by disrupting entrenched enemy positions, aimed to bypass strongpoints and to attack the weakest parts of an enemy's line. Additionally, they acknowledged the futility of managing a grand detailed plan of operations from afar, opting instead for junior officers on the spot to exercise initiative.
The Germans employed and improved infiltration tactics in a series of smaller to larger battles, each increasingly successful, leading up to the Battle of Caporetto against the Italians in 1917, and finally the massive German spring offensive in 1918 against the British and French. German infiltration tactics are sometimes called "Hutier tactics" by others, after Oskar von Hutier, the general leading the German 18th Army, which had the farthest advance in that offensive. After a stunningly rapid advance, the offensive failed to achieve a breakthrough; German forces stalled after outrunning their supply, artillery, and reinforcements, which could not catch up over the shell-torn ground left ruined by Allied attacks in the Battle of the Somme in 1916. The exhausted German forces were soon pushed back in the Allied Hundred Days Offensive, and the Germans were unable to organise another major offensive before the war's end. In post-war years, other nations did not fully appreciate these German tactical innovations amidst the overall German defeat.

Mining: Mines – tunnels under enemy lines packed with explosives and detonated – were widely used in WWI to destroy or disrupt enemy's trench lines. Mining and counter-mining became a major part of trench warfare.
The dry chalk of the Somme was especially suited to mining, but with the aid of pumps, it was also possible to mine in the sodden clay of Flanders. Specialist tunneling companies, usually made up of men who had been miners in civilian life, would dig tunnels under no man's land and beneath the enemy's trenches. These mines would then be packed with explosives and detonated, producing a large crater. The crater served two purposes: it could destroy or breach the enemy's trench and, by virtue of the raised lip that they produced, could provide a ready-made "trench" closer to the enemy's line. When a mine was detonated, both sides would race to occupy and fortify the crater.
If the miners detected an enemy tunnel in progress, they would often counter-mine and try to drive a tunnel under the enemy's tunnel in which they would detonate explosives to create a camouflet to destroy the enemy's tunnel. Night raids were also conducted with the sole purpose of destroying the enemy's mine workings. On occasion, mines would cross and fighting would occur underground. The mining skills could also be used to move troops unseen. On one occasion a whole British division was moved through interconnected workings and sewers without German observation. The British detonated 19 mines of varying sizes on July 1, 1916, the first day of the Battle of the Somme. The largest mines—the Y Sap Mine and the Lochnagar Mine—each containing 24 tons of explosives, were blown near La Boiselle, throwing earth 4,000 feet (1,200 m) into the air.
At 3.10 AM on June 7, 1917, a series of mines was detonated by the British to launch the Battle of Messines. The average mine contained 21 tons of explosive and the largest, 125 feet (38 m) beneath Saint-Eloi, was twice the average at 42 tons. As remarked by General Plumer to his staff the evening before the attack:

"Gentlemen, we may not make history tomorrow, but we shall certainly change the geography."
The craters from these and many other mines on the Western Front are still visible today. Two undetonated mines remained in the ground near Messines, with their location mislaid after the war. One blew during a thunderstorm in 1955; the other remains in the ground. Significant mining operations were also carried out on the Italian Front.

Gas: World War I saw large-scale use of poison gases. At the start of the war, the gas agents used were relatively weak and delivery unreliable, but by mid-war advances in this chemical warfare reached horrifying levels.
The first methods of employing gas was by releasing it from a cylinder when the wind was favourable. This was prone to miscarry if the direction of the wind was misjudged. Also, the cylinders needed to be positioned in the front trenches where they were likely to be ruptured by enemy bombardment. Later, gas was delivered directly to enemy trenches by artillery or mortar shell, reducing friendly casualties significantly. Lingering agents could still affect friendly troops that advanced to enemy trenches following its use.
Early on, soldiers made improvised gas masks by urinating on a handkerchief and putting it over their nose and mouth so the urea would disable the poison. Armies rushed to issue regulation gas masks as regular equipment for front line troops. Anti-gas equipment and procedures improved significantly during the war, to the point that gas attacks had become less devastating at the war's end.
Several different gas agents were used. Tear gas was first employed in August 1914 by the French, but this could only temporarily disable the enemy. In April 1915, chlorine gas was first used by Germany at the Second Battle of Ypres. Exposure to a large dose could kill, and those not killed could suffer permanent lung damage. But the gas was easy to detect by scent and sight. Phosgene, first used in December 1915, was the most lethal killing gas of World War I; it was 18 times more powerful than chlorine and much more difficult to detect.
However, the most effective gas was mustard gas, introduced by Germany in July 1917. Mustard gas was not as fatal as phosgene, but it was hard to detect and lingered on the surface of the battlefield, so could inflict casualties over a long period. Even if not inhaled, it could slowly burn the skin, but quickly burned via the eyes or any wounds, causing blindness and intense suffering.