A furuncle, commonly known as a boil, is a painful, pus-filled bump that forms under the skin when hair follicles become infected. This infection is usually caused by bacteria, particularly Staphylococcus aureus. Furuncles can occur anywhere on the body but are most often found in areas where friction or sweating occurs, such as the armpits, thighs, and neck. Furuncles typically start as red, swollen lumps and can grow larger over time. They may eventually rupture, releasing pus and relieving pressure. Treatment often involves warm compresses to promote drainage, and in some cases, medical intervention may be necessary to drain the boil or prescribe antibiotics.