Polycythemia (elevated red blood cell count) causes are either primary (aquired or genetic mutations) or secondary (diseases, conditions, high altitude).
Cancer is a disease caused by an abnormal growth of cells, also called malignancy. It is a group of 100 different diseases, and is not contagious. Cancer
Sickle cell anemia (sickle cell disease), a blood disease which shortens life expectancy, is cause by an inherited abnormal hemoglobin. Symptoms may include