Ten forms of suffering
The instances are more numerous than the deaths and injuries, because they extend to the families and friends of those hurt, often left in ignorance. The forms of the suffering were also varied. Here are ten (in sequence of occurrence not importance):
1. Devastation of house and home in the villages of Belgium and northern France: 6500 civilians there in the first three months
2. Dead or starving animals bellowing their distress
3. Blistered, choked or blinded by gas
4. The misery of lice infestation, rats, mud and trench foot
5. Killing by mortar shells, decapitation and bodily dismemberment
6. Drowning at sea – more than 140 ships sunk, including 25 hospital ships and the terrible losses of Lusitania and Mendi
7. Seeing comrades killed
8. Loss of limbs and disfigurement
9. ‘Shell shock’
10. General sense of desperation and futility.

The trench experience
https://www.bl.uk/world-war-one/articles/sensuous-life-in-the-trenches.
“As a result, 37 million were killed or wounded, delivering immeasurable grief to millions of families. At the mother of all battles, Arras, the British lost 159,000 men, a daily average of 4,076 killed, wounded and missing, the worst daily casualty rate of any major battle. Many who survived suffered wounds of unprecedented violence, rendering them unable to walk, see, hear or make love again.” Paul Ham historian
in Daily Telegraph 20 Feb 2018.

Gallipoli miseries
https://www.iwm.org.uk/history/9-reasons-why-gallipoli-was-one-of-the-worst-fighting-fronts-of-the-first-world-war.
Pictures of some of the horrors of WW1:
http://craigritchie.co.uk/archives/2581;
https://www.youtube.com/watch?v=OJTk32eUO3A; https://www.youtube.com/watch?v=OK6ZTXrEUfc;
https://www.youtube.com/watch?v=eVEaNg4eIjQ.

Rotting corpses, missing meaning
"'Coming out of the shelter, I realize that I'm walking or corpses. from the way the soil gives under my feet, and feels slippery and soft', wrote Captain La Chaussee, setting down a newcomer's disgust. Yet he arrived in  June, and by then thousands of men had already made the same discovery and felt the same disgust - including, quite possibly, the men whose corpses he now found himself walking over....
It was not that men died and their bodies rotted before disappearing into nothing. It was that here men died and their bodies were left to rot and disappear without the usual rituals, the decencies, of civilisation. The appalling indignity of their fate contained nothing to challenge the devout in their religious faith, however much it might disgust them or add to their grief. But it dealt a blow to men accustomed to expressing their devotion to France in terms borrowed from the language of religious faith." Ian Ousby The Road to Verdun (2002) pp 245-7

Sleeping sickness and tsetse fly
This was devastating in its impact on men and animals in the African campaigns in what was then German West Africa - now Tanzania, Ruanda and Burundi https://brighterworld.mcmaster.ca/articles/rare-map-sheds-light-on-deadly-perils-of-the-great-wars-forgotten-campaign With animals having four legs, they were at greater risk, even than men, especially so in swamp-land and river crossings. Having goods ported by Carriers reduced risk of bites - but not for them.

Deadly deception
Deceiving an enemy into thinking that the plan was to move in one direction when actually doing the opposite is a standard feature of competitive human behaviour past and present in both business, sport and war. Camouflaging of intent and realities on the ground was a regular feature in WW1. Specifically in trench warfare feints such as raisin empty helmets and even dummy heads with distinctive coverings such as turbans were common. Observation of where any sniping shots were coming from then permitted better targeted retaliation. An extreme example of deception involved the placing of a life-size military dummy that would rescuers who would become ready targets for snipers:
© IWM (Q 17688) "This dummy was used at Camouflage School, Kensington Gardens, to give an idea of a device and use of by the enemy on different parts of their front. Figures were made to appear like British soldiers by dressing them in captured Khaki. They were placed in No Man's Land and by means of a cord an arm was raised as if for assistance. Sometimes the body was made to sit up. Generally the head was bound up in bandages. The idea was to get our men to effect a rescue."

The painful ambiguity of Nursing and Munition making 
Suffering was as continuing feature both of wounded survivors and those in involved in their care. For an elaborated description of the complexity of this process: https://www.bl.uk/world-war-one/articles/experiences-of-a-wounded-soldier.
In relation to the Western Front, hospital provision was not only on either side of the English channel but distributed in local hospitals across the UK. One such is the Winwick Hospital near Warrington, which was requisitioned for military use as the Lord Derby War Hospital in 1915. Apparently it had its own railway siding (Liverpool - Manchester line) which was adapted to accept troops.
Up and down the country, nursing care by women in response to all manner of wounded suffering was largely taken for granted - as also was their fundamental role in munitions factories. 

https://www.iwm.org.uk/history/a-day-in-the-life-of-a-munitions-worker 
Treatment of Prisoners of War
https://www.bl.uk/world-war-one/articles/prisoners-of-war.