A pocket full of fluid on the skin that results due to excessive rubbing, freezing, burning, infection or chemical reaction is medically known as Skin Blister. Generally blisters are filled with fluid called plasma but in case of blood blisters the pockets are full of blood and in case of infected blisters pus fills the place of the fluid. This is normally an infection caused on the upper layer of the skin and the fluid onside the blister guards the other layers of the skin from further damage. Other medical causes are there that are responsible for happening of blisters. They are herpes, chicken pox, impetigo and certain types of eczema. Some other rare medical diseases are also there that also causes the blisters.