Boil (Old Eng. byl, Middle Eng. bile; akin to prov. Eng. and Scotch beal, 'to suppurate;') its present spelling having arisen through confusion with the verb boil, which is from the French) is a hard painful swelling of the skin. It usually begins as a small hard point of a dusky red colour, which is hot, painful, and throbbing. This point extends, and these symptoms increase in severity till sooner or later, when the boil ceases to enlarge, it is of a conical form, with a broad firm base, and on the apex a whitish blister, which contains a little matter; this opens, and after a few days more there is discharged the core, a little pellet of dead cellular tissue, and the small cavity left heals rapidly, leaving a white depressed scar. Sometimes boils subside without any breach in the skin, or any discharge. These are known as blind boils, and are generally less conical in shape, less painful, and slower in their progress than the common form. Boils rarely occur singly: very often the subjects of them suffer from successive crops during a period of some weeks or months. They may affect any part of the skin, except the palms of the hands and soles of the feet.
Boils are probably always caused by some abnormal condition of the blood or the nutrition of the body, though they often occur in people who appear to be in good health. They are most common in spring; are rare in young children, most frequent in older children and young adults. Any sudden change in diet and daily habits is very apt to cause them; thus persons undergoing a course of hydropathic treatment, or training for races, and country girls going into town as domestic servants, are very apt to suffer. They often occur also during recovery from exhausting diseases. 'In many cases they must be looked upon as salutary. After an outbreak of boils, the health often gradually improves' (Erichsen).
The best application to a boil in the early stages is collodion, frequently repeated, which often prevents its further development. Later, when the pain is severe, frequent bathing with hot water, or the application of small hot poultices, which may be made of poppy-heads, or of oatmeal or linseed-meal, will give relief and hasten the progress of the case. Incision is sometimes advisable if the patient's suffering is great. The constitutional condition must always be attended to, and any error in diet or habits corrected. Aperient medicines are generally useful, with, in enfeebled conditions, tonics; in plethoric conditions, alkalies. Sulphide of calcium, in doses of a tenth to a quarter of a grain, greatly hastens the ripening of boils, and prevents the formation of fresh ones.