If the boil is not lanced, it reaches its full development in seven to ten days with the formation of a central "core" of dead tissue and some pus, which gives to the center of the boil a whitish or yellowish-brown appearance. The boil then breaks down spontaneously in one or more places (usually only one) and discharges some pus, and, with a little pressure, also the white, central core of dead tissue. The remaining wound closes in and heals in a week or two. Boils occur singly or in numbers, and sometimes in successive crops. When this happens it is because the pus germs from the previous boils have invaded fresh areas of skin. =Causes.=--Boils are thus contagious, the pus germs being communicated to new points on the patient's skin, or to that of another person. Local irritation of the skin, from whatever cause, enables the germs to grow more readily. The existence of skin diseases, as eczema ("salt rheum"), prickly heat, and other sources of itching and scratching, is conducive to boils, as the pus germs contained in ordinary dirt are rubbed into the irritated skin. Whenever the skin is chafed by rough clothing, as about the wrists and neck by frayed collars and sweaters, etc., boils are likely to occur. Also when the face and neck are handled by barbers with dirty hands or instruments, a fruitful field is provided for their invasion. While boils are always the result of pus germs gaining entrance to the skin glands, and, therefore, strictly due to local causes, yet they are more prone to occur when the body is weakened and unable to cope with germs which might do no harm under other circumstances. The conditions favoring the occurrence of boils are: an impoverished state of the blood, errors of diet and indigestion, overwork, dissipation, and certain diseases, as typhoid fever, diabetes, and smallpox. Boils are thought to occur more frequently in persons with rough skin and with a vigorous growth of dark hair. They may be situated on any part of the body, but certain localities are more commonly attacked, as the scalp, the eyelids, cheeks, neck, armpits, back, and buttocks. Boys and young men are generally the sufferers. =Treatment.=--The importance of cleanliness cannot be overestimated in the care of boils if we keep their cause in mind. Dirty underclothes or fingers used in squeezing or otherwise handling the boil, may carry the trouble to fresh parts. Any sort of local irritation should be removed; also all articles of clothing which have come in contact with the boils should not be worn until they have been washed in boiling water. There is no single remedy of much value for the cure of boils, although pills of calcium sulphide (each one-tenth grain) are commonly prescribed by physicians, every three hours.