Trench foot, formerly known as chilled foot until it became a widespread malady in the Great War, occurs when the feet are wet for long periods of time. It can be quite painful, but it can be prevented and treated. Symptoms of trench foot include a tingling and/or itching sensation, pain, swelling, cold and blotchy skin, numbness, and a prickly or heavy feeling in the foot. The foot may be red, dry, and painful after it becomes warm. Blisters may form, followed by skin and tissue dying and falling off. The photo above shows an extreme case.
How did the military deal with Trench foot? Here's some documentation: