Disease - How it works

Classifying Diseases

Any condition that impairs the normal functioning of an organism can be called a disease. In the human organism, as in all others, there are certain basic requirements, which in the human body include the need for a certain proper amount of oxygen, acidity, salinity (salt content), nutrients, and so on. These conditions must all be maintained within a very narrow range, and any deviation can bring about disease.

Diseases can be classified into three general groups. There are conditions that are infectious, or extrinsic, meaning that they are caused by an infection through which a virus, bacterium, or other parasite enters the body. Infectious diseases, infections, and the immune system that usually protects us against them are discussed elsewhere in this book. Our attention in the present context will be devoted to the other two broad categories—noninfectious, or intrinsic, diseases and diseases of unknown origin.

CLASSIFYING INTRINSIC DISEASES.

There are several basic varieties of intrinsic disease, or conditions that are neither contagious nor communicable. These varieties are listed in the next few paragraphs. The essay Noninfectious Diseases includes a discussion of other systems for classifying diseases of either the intrinsic or the extrinsic variety.

Hereditary diseases: diseases that are genetic, meaning that they are passed down from generation to generation. An example, discussed in Noninfectious Diseases, is hemophilia. Heredity is not a "cause," and some of the diseases of unknown origin may be transmitted from parent to offspring. Some forms of cancer are hereditary as well, as are other conditions discussed elsewhere in this book. (See Nonifectious Diseases, Mutation, and Heredity.)

Glandular diseases: Conditions involving a gland—that is, a cell or group of cells that filters material from the blood, processes that material, and secretes it either for use again in the body or to be eliminated as waste. Examples include diabetes mellitus, examined in Noninfectious Diseases, as well as various kidney and liver diseases, among them, hepatitis and jaundice. Goiter, a swelling in the neck area caused by a diet poor in iodine, is both a glandular and a dietary condition, a fact that illustrates the overlap between disease types.

Dietary diseases: These are all illnesses that relate to nutrient deficiencies—either an overall lack of adequate nutrition (i.e., malnutrition) or the absence of a key nutrient. Examples include pellagra, scurvy, and rickets, all of which are vitamin deficiencies, as well as kwashiorkor, which brings about a swollen belly and is caused by a lack of protein. Vitamin deficiencies are discussed in Vitamins, and kwashiorkor and other varieties of malnutrition are examined in Nutrients and Nutrition.

Cancers: Cancer is not just one disease but some 100 conditions. Its two main characteristics are uncontrolled growth of diseased cells in the human body and migration of the disease from the original site to distant sites within the body. If the spread is not controlled, cancer can result in death. (See Noninfectious Diseases for more.)

Stress-related diseases: Some heart conditions are hereditary or glandular, but quite a few diseases of the heart and circulatory system are exacerbated by stress. Examples include heart murmurs, hardening of the arteries, and varicose veins. We will examine heart disease and the general effects of stress shortly.

Autoimmune diseases: This is a particularly terrifying category of disease, because it involves a rejection of the body itself by the body's own immune system. Autoimmune diseases, examples of which include lupus and rheumatoid arthritis, are discussed in The Immune System.

DISEASES OF UNKNOWN ORIGIN.

Finally, there are diseases for which there is no known cause. In some cases, it is possible that heredity, diet, or some other aspect of human existence has a role, but it is not certain. And even if, say, heredity plays a part, the exact hereditary factors are not established. In any case, many of the categories of disease we have listed do not amount to "causes," but rather are types of disease. Moreover, some diseases classifiable in one of the listed categories also belong in the ranks of the diseases with unknown causes. For instance, many autoimmune diseases are mysterious to scientists. Likewise, chronic fatigue syndrome, considered a disease of unknown origin, is obviously a stress-related disorder, while fibromyalgia, characterized by sore muscles and tissues, may be stress-related as well. Two brain diseases of unknown origin, Creutzfeldt-Jakob and Alzheimer's disease, are discussed near the conclusion of this essay.