polycythemia vs blood disease

polycythemia and blood disease both are nouns.

Word	Noun	Adjective	Verb	Adverb
polycythemia	Yes	No	No	No
blood disease	Yes	No	No	No

As nouns, blood disease is a hypernym of polycythemia; that is, blood disease is a word with a broader meaning than polycythemia:

polycythemia: a disorder characterized by an abnormal increase in the number of red blood cells in the blood
blood disease: a disease or disorder of the blood

Other hypernyms of polycythemia include blood disorder.

polycythemia (noun)	blood disease (noun)
a disorder characterized by an abnormal increase in the number of red blood cells in the blood	a disease or disorder of the blood