A hematologist is a doctor in a specific branch of medicine that deals with blood and who is able to diagnose and treat a variety of blood diseases and disorders. The word "hematologist" and the name of the medical branch, "hematology," originate from the Greek word "heme," which means "blood."
As a hematologist, a physician must go through a few years of learning how to diagnose disorders. Doing so gives a hematologist expertise on abnormalities in the blood, diagnosis of illnesses such as blood cancer and the ability to treat a wide range of disorders that may affect the blood. They can also perform blood transfusions.