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).
Blisters are a very common cause of foot pain symptoms. They occur frequently on the bottom of the feet but can also be seen on the top and side of the foot. In the foot at least, the primary cause is friction. The most common cause is a poor fitting shoe combined with an activity that creates excessive friction on a certain part of the foot. You can also get foot blisters from eczema and athletes foot. In addition they can occur from burns and situations where the feet are too moist thus increasing friction.
From the excessive friction the upper layer of skin, the epidermis, separates from the middle layer of skin, the dermis. Fluid fills in and the blister is formed. Initially the lesion is formed to actually protect an area but more often than not on the foot the blister will begin to hurt.