About.com: Rare Diseases (Definition)
Polycythemia vera (PV) is a bone marrow disorder (myeloproliferative disorder) in which there is an overproduction of red blood cells. Production of white blood cells and platelets is also increased in most individuals. Polycythemia vera occurs in about 1 in 100,000 individuals, most often those aged 50-70 years, but may occur at any age. It affects both men and women of all ethnic backgrounds.