Boil. A furuncle, also known as a boil, found on the upper thigh of a patient. This skin infection is usually caused by the bacterium Staphylococcus aureus which may enter through a hair follicle or a break in the skin. A boil starts as a red painful lump which progressively fills with pus. A hot compress applied every two hours should relieve discomfort and hasten possible drainage and healing. If a boil is large and very painful it is treated with antibiotics or in severe cases may be lanced by a doctor to allow the pus to drain.