Below is an instruction from Human. Write a response.
    ### Instruction:
    What is (are) Polycythemia vera ?
    ### Response:
    Polycythemia vera (PV) is a condition characterized by an increased number of red blood cells in the bloodstream. Affected people may also have excess white blood cells and platelets. These extra cells cause the blood to be thicker than normal, increasing the risk for blood clots that can block blood flow in arteries and veins.
    