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A nevus, or naevus (the Latin word for mole, adopted into English in the late 1600s by doctors), is usually a benign spot or raised area on the skin that is a different, usually brown but sometimes a reddish or purplish color. They are caused by nevomelanocytes, a type of pigment-producing cell that is somewhat different from the melanocytes which control the overall color of the skin, and can be congenital or grow later in life. However, no one knows why the nevomelanocytes form. There are numerous kinds of nevi, including a few rare types that form inside parts of the body rather than on the skin.

Smaller ones are very common and usually just called moles or birthmarks and ignored; it's when large ones form that there are concerns about getting rid of them. (Large is usually defined as bigger than 8 inches/20 cm across on an adult, or 3 inches/8 cm on an infant.) Laser treatment and chemical peels can be used to lighten nevi if the patient just doesn't want them visible for aesthetic reasons; surgery is usually required to get rid of them entirely, using skin grafts to cover the exised area.

Large nevi can have a noticeable dry, leathery feel compared to the rest of the skin (due to lack of normal oil glands), and the skin of a nevus generally does not heal as easily as normal skin. Scalp nevi often make the texture of the hair growing from that part of the head a different texture from the rest of the head's hair. Sweat glands do not form normally in them, so the area does not sweat; sometimes the person with a nevus over a large area of their body will sweat more over the rest of the body to lose the necessary amount of heat. Large nevi may increase one's risk of skin cancer, and rapid changes in any mole, small or large, should be checked out for melanoma. On the uncommon occasions when the nevus cells get into the nervous system, they can cause a condition called neurocutaneous melanosis which causes neurological problems, for which there is very limited treatment and no known cure.


Nevus also seems to be the name of

Sources:
http://www.nevus.org/about/aboutnevi.html
http://www.nevusnetwork.org/nev_info.htm
http://www.drhull.com/EncyMaster/N/nevus.html
http://radiology.uchc.edu/eAtlas/Skin/508.htm
http://dictionary.oed.com/
http://wordsmith.org/awad/archives/0802
http://www.nevus.com/
http://www.nevusband.com/

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