Common causes of pus in blisters are bacterial infections that are minor in most cases but can sometimes lead to further complications. Most types of pus result from a purulent infection where the immune system is triggered to fight harmful bacteria. Pus in blisters is frequently found in surface skin conditions such as acne, folliculitis, minor burns, or cold sores resulting from the Type I herpes simplex virus. Other causes of pus in the inner body tissues include strep throat, tooth abscesses, or infected tonsils.

Mild burns on the skin often form fluid-filled blisters as a result of damage to underlying tissue and small blood vessels. The liquid inside one of these sores is known as serum, and it functions as a protective layer for the injured tissue. While draining a blister is a common practice with one of these burns, it can introduce bacteria that leads to the formation of pus cells. When the body's immune cells attack invading bacteria, pus results when these cells disintegrate and die off. The cells that form pus in blisters are called neutrophils, and they eventually break down even after successfully countering the infectious bacteria.

Pus in blisters frequently occurs in cases of acne when sufferers pop or squeeze the blemishes. Dermatologists often advise against this practice because it can worsen the appearance of an initial blemish as well as increase the chances of infection and scarring. Folliculitis is an infection of the hair follicles that results in small pus-filled blisters at the point where the hair first grows out from the scalp or other skin surface. Cold sores that have been opened also commonly become causes of pus in blisters due to the introduction of bacteria. Any of these skin conditions usually heal at better rates when sufferers wash their hands frequently, apply topical antibacterial medicines, and refrain from otherwise touching the sores.

Internal infections can also lead to the formation of pus-filled blisters. This problem is common in throat infections such as strep and tonsillitis. Tonsils that regularly break out in blisters with pus can be especially troublesome and usually require surgery to completely eliminate this condition. Dental abscesses occur from bacterial invasion in a tooth canal and can be another cause of painful mouth sores filled with pus. While these types of health conditions are unpleasant, they are usually cleared up with routine surgical procedures and antibiotic medications.