Trench warfare
The trenches were dirty.  Some men disappeared into the mud because it was so thick.  The cold, wet and unsanitary conditions made many soldiers sick.  Lice spread trench fever, a fungal infection which caused severe head pain and fever.  Rats invaded the trenches and spread disease everywhere.  The brown rats were the more hated kind.  They ate human remains.  Some grew to be as big as cats.