A boil is a painful, pus-filled bump that forms under the skin when bacteria infect a hair follicle or oil gland. It usually starts as a red, swollen area and can grow larger over time. Boils can occur anywhere on the body but are most common on the face, neck, armpits, and thighs. The primary cause of boils is the bacterium Staphylococcus aureus, which can enter the skin through small cuts or abrasions. Treatment often involves warm compresses to help drain the boil, and in some cases, a doctor may need to perform a minor procedure to remove the pus.