A medical doctor who specializes in studying and treating blood diseases, blood disorders, and diseases of the bone marrow.
From Greek 'haima' (blood) + 'logos' (study) + '-ist' (one who practices). The term emerged as blood medicine became specialized enough to require dedicated doctors.
Modern hematologists can look at a single drop of blood and identify dozens of diseases—from infections to cancers—which is why they're sometimes called 'blood detectives' in hospitals.
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