Diagnosis and disease are not the same. A diagnosis is a practical definition of a disease. A disease on the other hand is best considered a conceptual approach to a dysfunctional body system. Some diseases affect more that one body system.
A diagnosis is based on signs and symptoms, things that can be identified and demonstrated. The criteria for diagnosis are based on standards such as the ICD classification. This would include physical signs such as swelling or discoloration of the skin and symptoms such as pain. A diagnosis is frequently supported by one or more tests. A diagnosis allows us to move forward with treatment, and make decisions including mitigation strategies such as isolation. A diagnosis usually implies or requires a certain treatment and conversely treatments may require or at least are best for a certain diagnosis. An important aspect, of a diagnosis is that it allows providers to bill patients and insurance companies. There is a certain finality to a diagnosis and there is often a commitment to our impression especially after treatment has begun.
A disease is a conceptual or theoretical assertion that there is infection, neoplasia (cancer), degeneration due to aging or other factors, diseases of heredity, or the environment including injury or exposure as in poisoning. Diseases are described in these terms and not so rigorously categorized as are diagnosis. The idea of a disease is better suited to the consideration of pathophysiology, the causes and progression of disease and not necessarily any particular treatment. When we consider that a disease is present we also recognize that there is always more or less uncertainty as to its nature. This allows us to continually re-evaluate our understanding of it. The usefulness of the characterization of diseases is in research and education.
Heart Failure is a diagnosis with specific codes for various types. It is based on signs and symptoms. Cardiovascular Insufficiency is the corresponding disease and signifies that the flow of blood supply does not meet demand.