hypertrichosis lanuginosa can be
aquired or
congenital and is characterised by excessive growth of body hair. The term
lanuginosa derives from '
lanugo hair' which is the very first hair made by the
follicles, which actually happens inside the
womb.
The aquired version is often a sign of a cancer, and has been known as malignant down. (Really the hair in the adult shouldn't be described as lanugo, but rather as vellus hair.) Most often it's facial hair that's affected, but it can disrupt any bodily hair growth. It's thought that the malignant, cancerous tissue can disrupt hormone production, or maybe produce a cytokine based hair growth stimulator.
With the congenital version, the excess hair growth is of course present from birth, and persists thoughout the individual's life. As the lanugo hair is very fine and unpigmented an afflicted person appears to covered in a light coloured fur! (NB only about 1 in 1,000,000 of the population are affected.)
The hypertrichosis lanuginosa should be contrasted with a variant, hypertrichosis terminalis where the hair is pigmented.