Skin disorders are diseases that occur mainly on the skin itself. However, they can also affect the sebaceous and sweat glands, hair or nails. The skin reacts to external stimuli, such as pathogens, cold, heat, radiation, toxins or drugs. However, internal processes such as hormonal changes or an autoimmune disease can also cause skin changes. Some skin conditions can be unsightly but harmless, others may be contagious. A lot of skin disorders are also itchy or even painful.