A blister is a small pocket of fluid within the upper layers of the skin, typically caused by forceful rubbing (friction), burning, freezing, chemical exposure or infection. Most blisters are filled with a clear fluid called serum or plasma. However, blisters can be filled with blood (known as blood blisters) or with pus (if they become infected). The word "blister" entered English in the 14th century. It came from the Middle Dutch "bluyster", and was a modification of the Old French "blostre" which meant a leprous nodule—a rise in the skin due to leprosy.