Illustration of polycythemia vera, a rare blood cancer with an increase in the number of red blood cells in the body due to mutation in the JAK2 gene. These excess cells thicken blood, slowing its flow. They also cause complications, such as blood clots, which can lead to a heart attack or stroke. Without treatment, polycythemia vera can be life-threatening. Medical care can help ease signs, symptoms and complications of this disease. Over time, in some cases there's a risk of progressing to more-serious blood cancers, such as myelofibrosis or acute leukemia. |