Collective name for various malignant skin diseases. Most skin tumours are benign, but it is wise to consult a doctor if a birthmark, mole or other protrusion on the skin grows, changes shape or begins to bleed or itch. A birthmark can develop into a melanoma, and basal cell carcinoma can form in skin areas exposed to the sun for long periods (ultraviolet light); this form of skin cancer fortunately does not metastasize, in contrast with a melanoma and spinal cell carcinoma. In the last two cases in particular, early diagnosis is important so that the tumour can be removed by surgery or treated by radiation. Disorders of the skin can be divided into a number of categories, both in form and cause. Eczemas form the largest group. Other large groups are tumours and infections. Other skin conditions are grouped according to certain common features, such as the formation of blisters. Skin conditions have many causes; a number are allergies – contact eczema for example – or skin eruption following the use of certain medicaments. Many skin conditions are caused by viral infection, from warts and cold-sores to the red spots of measles or German measles. Bacteria can produce skin disorders in two ways, first by themselves causing inflammation in the skin (impetigo, barber’s rash) or in the sebaceous glands (acne), the hair follicles (folliculitis, boil) or the sweat glands (hidradenitis). The second is by producing substances that cause the skin disorder (scarlet fever). Is is also possible for skin disorders to be caused by parasites, such as lice or mites (scabies, for example), fleas and other insects. Skin tumours can be the result of exposure to radiation (especially ultraviolet light), a developmental disorder or accumulation of certain substances in the skin (xanthomas). Sometimes absolutely no cause can be found. Skin eruptions can be divided into three categories. The first are on the same level as the skin, and include changes of colour, such as redness, too little or too much pigment, vascular dilation in the skin and discharge of blood in the skin (purpura). The second category are above the level of the skin, including all lumps from less than 1 mm (pimples) to larger than 1 cm (tumours) in diameter, and also blisters. Urticaria (nettlerash) occurs when the skin becomes locally thicker and paler, because of fluid between the skin cells. The third category includes conditions below the normal level of the skin, retractions of the skin, erosions (abrasion), wounds and ulcers. All skin disorders consist of one or more of these elements, often in combination, such as redness with pimples, or an ulcer with a red edge.