A boil is caused by a bacterial skin infection. This skin abscess forms deep inside a hair follicle or oil gland. A boil generally starts as a reddened, tender area. Over time, the area becomes firm and hard.

The infection damages your skin cells, hollowing the tissue out. Your immune system responds with white blood cells, which fill the center of the infection and make it soft. Your body makes these cells to destroy the infection.

Together with bacteria and proteins, these white blood cells are known as pus. This pus may eventually form a central head near the surface of your skin. This head may drain on its own, spilling out of the surface of your skin. If not, it can be surgically opened.