Answer:
Trench foot
Step-by-step explanation:
Trench is a defensive line that soldiers can dug within the ground where they can slip their body in to protect themselves from upcoming bullets and artilleries.
Soldiers that were assigned in the trench had to stand long hours for days or months while paying attention to the potential attacks from across the defensive line.
Since their foots were under a lot of pressures from standing for a long period of time in somewhat unsanitary area, the skin can break down and start to form an infection. This condition was so common among trenches to the point where it become widely used to describe the conditions in the trenches.