Internal factors: As the first physiological defense and the largest organ of the human body, the skin is always involved in the functional activities of the organism and maintains the unity of the organism and the natural environment in opposition to each other, and any abnormalities of the organism can also be reflected on the skin surface, as well. For example, psoriasis, psoriasis, vitiligo, allergic itchy rash, lupus erythematosus, skin manifestations of visceral cancer, etc., are both indirectly or directly related to abnormal cell division, infection by pathogenic microorganisms and the toxins they produce, disorders of body metabolism, imbalance of immune function, endocrine disorders, free radical toxin metabolism disorders, and even pathological changes in the mental and nervous systems. External causes: Mechanical: such as callus, abrasive erythema, trauma, etc. Physical: e.g. frostbite, burns, sunburns, radiation dermatitis, etc. Chemical: Most contact dermatitis is caused by exposure to chemicals, such as dyes, chemicals, etc. Biological: Animal, such as scabies, insect bite dermatitis, leech bites, poisonous fish stings, etc. Plant, such as contact with lacquer trees, nettles, etc. Microorganisms such as bacteria, germs, molds, spirochetes, etc. Many skin diseases that continue to develop or persist even after the cause is removed may be due to the constant action of other stimuli such as scratching, abrasion (mechanical), hot water scalding, sun exposure (physical), soap washing, improper medication (chemical) and alcohol consumption. Among them, scratching is the most prominent and important. Due to the mechanical stimulation of frequent scratching, the damage is often thickened or enlarged, and the disease persists. Conversely, if you stop scratching, chronic eczema, pruritus, or neurodermatitis, which are not easily cured, can often be healed more quickly. In addition, age, gender, occupation, season, environment, living habits and other factors also have a certain relationship with the occurrence of skin diseases.