Trench foot is a medical condition caused by prolonged exposure of the feet to damp, unsanitary, and cold conditions. It is one of many immersion foot syndromes. The use of the word "trench" in the name of this condition is a reference to trench warfare, mainly associated with World War I.
When Sgt. Alistair Walsh finally deployed into the field during the Battle of France, he found conditions even worse than he had remembered from World War I. Trench foot was a problem but Walsh recalled a trick from the previous war, namely to rub Vaseline on your feet as thick as you could. He kept after his men and so only one soldier came down with trench foot.