The main big one would be disease and infections. Trench fever, trench foot and trench mouth were common diseases people got when involved in trench warfare. Trench foot happens when the feet are exposed for long periods of time to cold water and mud. Trench fever is caused when body ice transfers from one place to another. Overcrowding conditions make this prone to happen. It causes headaches, sore muscles, skin lesions on the chest and backs as well as fever.
I hope this helps :)
