A blister is an ulceration or fluid filled pocket on the skin, generally caused by pressure and friction. They are common on the feet, particularly in persons who walk in ill-fitting or inflexible footwear (such as high heels).
Blisters are also common with Radiation sickness and appear at the exposure site.
Fluid filled blisters should never be broken, as this will expose the patient to opportunistic infection, such as Staphylococcus. First aid is to cover the blister with a sterile bandage and secure it well. Blisters will usually resolve themselves in about 7 days.
Blisters that appear to be infected, or those that are very painful, should be examined by a physician.