The number of known dead has been placed at about 10,000,000 men. The main method of combat during the first world war a.k.a. the Great World War, was trench warfare. Trench warfare was one of the main reasons so many men died. It was a ruthless system of warfare, in which lines and lines of men were repeatedly mowed down, one after the other. .
Life in the trenches, on the daily, was filled with horror, and death. Death was a constant companion to those serving in the line, even when no raid or attack was launched or defended against. In busy sectors the constant shellfire directed by the enemy brought random death, whether their victims were lounging in a trench or lying in a dugout (many men were buried as a consequence of large shell-bursts). Similarly, novices were cautioned against their natural inclination to peer over the parapet of the trench into no man's land. Many men died on their first day in the trenches as a consequence of a precisely aimed sniper's bullet. It has been estimated that up to one third of allied casualties on the Western Front were actually sustained in the trenches. Aside from enemy injuries, disease wrought a heavy toll.
The trenches were also a place full of disease, a part played largely by the rats. .
Rats in their millions infested trenches. There were two main types, the brown and the black rat. Both were despised but the brown rat was especially feared. Gorging themselves on human remains, they could grow to the size of a cat.
Men, exasperated and afraid of these rats, would attempt to rid the trenches of them by various methods: gunfire, with the bayonet, and even by clubbing them to death.
Rats were by no means the only source of infection and nuisance though. Lice were a never-ending problem, breeding in the seams of filthy clothing and causing men to itch unceasingly. Even when clothing was periodically washed and deloused, lice eggs invariably remained hidden in the seams; within a few hours of the clothes being re-worn the body heat generated would cause the eggs to hatch.