The consideration of the cause of disease, as they individualize the dermatoses, will be found under their respective heads. At this place, however, a brief discussion of the more common and important etiological influences will be given. Causes may be divided for convenience into the predisposing which are generally of an internal origin, and the exciting or direct which are of external origin. Predisposing causes include inherited conditions of the system or acquired conditions which may be manifested by the presence of general or local internal disease. Eruptions arising under such conditions may be incidental but they are usually essential and hence are called symptomatic skin diseases. Among these may be mentioned the exanthemata, diseases due to constitutional causes or infections as syphilis, leprosy and purpura, or those following the ingestion of drugs, drug eruptions. These conditions are symptomatic of a constitutional agent, whether it be known or unknown, bacterial, chemical or toxic. They may be caused directly by some underlying disturbances or indirectly through some action on the nervous system or the vasomotor apparatus.
Exciting or direct causes are often of external origin and include those agencies which act directly on the skin itself, actually or apparently. Diseases produced in this manner, together with those originating in or confined to the skin, even if their causes are not determined, are designated idiopathic skin diseases. The local parasitic diseases, many hypertrophies and dermatitis venenata are examples of this group. It is well to remember, however, that in many instances the local causative factors in idiopathic diseases are influenced or even made possible by underlying conditions of organic disturbances. It is also possible that the same disease at one time may be symptomatic and at another time, idiopathic. The direct relation between internal diseases and the cutaneous disturbances, apparently caused by them, is often difficult to explain. In a measure this may be due to the activity of one or many factors operating at the same time to produce diseased conditions of the system, of separate organs or of parts not easily isolated or defined.
Climate and Seasons are well known causative factors. It is probable, however, that they alone are not so productive of disease as their combination with habits of living and other personal factors. Nevertheless, it is a fact that prurigo and lupus vulgaris are common in Austria, favus and pellagra in Italy, gangosa in Guam, rosacea in England and Germany and leprosy in many widely scattered points. The endemic feature of many diseases is well understood. Warm weather favors outbreaks of urticaria, miliaria and intertrigo, and cold weather aggravates psoriasis, seborrhoea, lupus vulgaris, ichthyosis and pruritus. Chilblains and many forms of dermatitis originate in cold weather while furuncules and the erythemata are most frequent in the spring and autumn. Sudden changes in temperature may cause greater activity in many existing eruptions or a fresh outbreak of the same condition. Diseases behaving in this manner are called recurrent when there appears to be some definite course.
Abode or Habitation. If unhygienic, these factors may contribute their quota in disease-giving influences. Thus the air of houses and rooms, polluted with sewer gas and other noxious emanations, are favorable conditions for the occurrence of pemphigus (in infants), furuncles, strumous and catarrhal diseases; and associated with uncleanliness of the person, largely diminish the resistance of the skin to the invasion and multiplication of animal and vegetable parasites.