Diagnosis, prognosis should perhaps be called near synonyms; but they can be confused because both are employed in a specific medical sense with clear, sharp implications that are often carried over into their general and extended use.
Diagnosis applies to the act or art of recognizing or of identifying a disease or diseased condition by analysis of such factors as the history of the case, its subjective symptoms, and its objective signs as revealed by observation or by special laboratory tests (as a count of the blood cells or an X-ray examination).
Prognosis applies to the act or art of foretelling the course and the termination of a disease; the term usually implies a correct diagnosis and knowledge of how the disease will affect the patient as it runs its course and of how it will end.