Classification of disease had helped to ferret out an important, frequently causal, relationship.

The most widely used classifications of disease are 

(1) Topographic, by bodily region or system, 

(2) Anatomic, by organ or tissue, 

(3) Physiological, by function or effect, 

(4) Pathological, by the nature of the disease process, 

(5) Etiologic (causal),

(6) Juristic, by speed of advent of death, 

(7) Epidemiological, and 

(8) Statistical. 

Any single disease may fall within several of these classifications.

In the (1) Topographic classification, diseases are subdivided into such categories as gastrointestinal disease, vascular disease, abdominal disease, and chest disease. 

In the (2) Anatomic classification, disease is categorized by the specific organ or tissue affected; hence, heart disease, liver disease, and lung disease. 

The (3) Physiological classification of disease is based on the underlying functional derangement produced by a specific disorder. Included in this classification are such designations as respiratory and metabolic disease.
Respiratory diseases are those that interfere with the intake and expulsion of air and the exchange of oxygen for carbon dioxide in the lungs.
Metabolic diseases are those in which disturbances of the body's chemical processes are a basic feature.
Diabetes and gout are examples.

The (4) Pathological classification of disease considers the nature of the disease process.
Neoplastic and inflammatory disease are examples.
Neoplastic disease includes the whole range of tumors, particularly cancers, and their effect on human beings.

The (5) Etiologic classification of disease is based on the cause, when known.
This classification is particularly important and useful in the consideration of biotic disease.
On this basis disease might be classified as staphylococcal or rickettsial or fungal, to cite only a few instances.
It is important to know, for example, what kinds of disease staphylococci produce in human beings.
It is well known that they cause skin infections and pneumonia, but it is also important to note how often they cause meningitis, abscesses in the liver, and kidney infections.
The sexually transmitted diseases syphilis and gonorrhea are further examples of diseases classified by etiology.

The (6) Juristic basis of the classification of disease is concerned with the legal circumstances in which death occurs.
It is principally involved with sudden death, the cause of which is not clearly evident.
Thus, on a juristic basis some deaths and diseases are classified as medical-legal and fall within the jurisdiction of coroners and medical examiners.
A person living alone is found dead in bed--dead of natural causes or killed?
Had the person who dropped dead on the street been given some poison that took a short time to act?
Much less dramatic, but perhaps more common, are disease and death caused by exposure of the individual to some unrecognized danger to health in working or living conditions.
Could the illness or disease be attributable to fumes or dusts in a factory?
These are examples of the many types of disease and death that fall properly in this classification.

The (7) Epidemiological classification of disease deals with the incidence, distribution, and control of disorders in a population.
Epidemiology is one of the important sciences in the study of nutritional and biotic diseases around the world.
The United Nations supports, in part, the World Health Organization, whose chief function is the worldwide investigation of the distribution of disease.
In the course of this investigation, many observations have been made that help to explain the cause and provide approaches to the control of many diseases.

The (8) Statistical basis of classification of disease employs analysis of the incidence (the numbers of new cases of a specific disease that occur during a certain period) and the prevalence rate (number of cases of a disease in existence at a certain time) of diseases.
If, for example, a disease has an incidence rate of 100 cases per year in a given locale and, on the average, the affected persons live three years with the disease, it is obvious that the prevalence of the disease is 300.
Statistical classification is an additional important tool in the study of possible causes of disease.
These studies, as well as epidemiological, nutritional, and pathological analyses, have made it clear, for example, that diet is an important consideration in the possible causation of arteriosclerosis.
The statistical analyses drew attention to the role of high levels of fats and carbohydrates in the diet in the possible causation of arteriosclerosis.
The analyses further drew attention to the fact that certain populations that do not eat large quantities of animal fats and subsist largely on vegetable oils and fish have a much lower incidence of arteriosclerosis.

Thus, statistical surveys are of great importance in the study of human disease.

Medical and Alternative Medical Terminology, Explanations -  by Courtesy of Encyclopedia Britannica® 2001