A diagnosis and a disease are not the same. 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. This is always applied to conditions that are considered alterations of normal physiology and not just variations of normal.
A diagnosis is a practical assignment for a particular patient. This assignment is made by a number of criteria or test results that should and usually are accurate. It is possible that this assignment is incorrect for the disease ostensibly referred to. This is known as a false positive usually because of a test that gives the incorrect result.
The criteria used to formulate and assign a diagnosis are usually signs and symptoms. These are tangible things that can be identified and demonstrated. 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 such a the analysis of blood. A diagnosis allows a health care provider to move forward with treatment, and make decisions including mitigation strategies such as isolation. It should be kept in mind that a diagnosis can be incorrect. A false positive test is a common problem. A diagnosis allows us to move forward given a reasonable degree of certainty. If the treatment to be employed is either expensive or invasive then a greater degree of accuracy is necessary. A diagnosis usually implies or 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. The ICD classification specifies the appropriate criteria for diagnosis that would be used in billing. There is a certain finality to a diagnosis and there is often a commitment to our impression especially after treatment has begun.
Diseases are conditions of body structure and function that are not normal. Diseases can be categorized as to the cause. These can be infection, neoplasia (cancer), injury (trauma) including poisoning or toxic exposure. Anatomical or physiological deterioration due to age or other factors can be abnormal if accelerated and falls outside the range of normal. The idea of a disease is better suited to the consideration of pathophysiology, the causes and progression of illness or injury. 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. Without the need for billing medical services and ICD categorization we have greater flexibility to be more accurate in the characterization of diseases.