Disease (or "lack of ease") is any damage or injury that impairs an organism's function. Diseases (sometimes called deviations from the norm) can be classified in numerous ways. Generally, an acute disease comes on quickly and lasts for only a relatively short time. A chronic disease usually begins slowly and lasts for a longer time.

Diseases can also be classified according to type. Common disease types or categories include: infectious, genetic (hereditary), psychiatric, deficiency, degenerative, congenital (whether genetic or not), neurological, cardiovascular, metabolic, chemical, and occupational.

Infectious or microbial diseases (the pathogenic diseases) are often classified by their causative agents: bacteria, fungi, protozoans, viruses, or prions. Progressive diseases, particularly those caused by microbes, have several clinical stages: infection, incubation, acute, decline, and convalescent. "Prodromal" refers to the initial stages when perhaps only one or two early characteristics of the disease can be observed. Communicable diseases are transmitted either directly or indirectly (via carriers or vectors ) from one organism to another. Contagious diseases are rapidly transmitted infectious diseases. Malignant diseases usually progress quite rapidly and are potentially life threatening.

In contrast to disease (the deviation itself), "illness" is feeling of being sick, or suffering some effect of the disease. Many people have hypertension (the disease), for example, without feeling any illness until it is so far advanced that is causes a stroke or kidney failure. This is why hypertension is sometimes called "the silent killer."

SEE ALSO Autoimmune Disease ; Bacterial Diseases ; Cardiovascular Diseases ; Genetic Diseases ; History of Medicine ; Homeostasis ; Neurologic Diseases ; Parasitic Diseases ; Psychiatric Disorders, Biology of ; Sexually Transmitted Diseases ; Viral Diseases

Roberta M. Meehan


Madigan, Michael T., John M. Martinko, and Jack Parker. Brock Biology of Microorganisms, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2000.

Thomas, Clayton L., ed. Taber's Cyclopedic Medical Dictionary, 18th ed., Philadelphia, PA: F. A. Davis Company, 1997.

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