Definition of Polycythemia vera:

Polycythemia vera is a bone marrow disease that leads to an abnormal increase in the number of blood cells (primarily red blood cells).

Causes, incidence, and risk factors:

Polycythemia vera is a disorder of the bone marrow. It mainly causes too much production of red blood cells, although the numbers of white blood cells and platelets are also increased.

It is a rare disease that occurs more often in men than women, and is rare in patients under age 40. It is usually associated with a gene mutation called JAK2V617F. The cause of this mutation is unknown.