Disease is a departure from the normal state of health, in which a delicate balance between chemical, physical, and functional processes is maintained. This homeostatic state is supported by a variety of control mechanisms, some of which are very complex. Disease occurs when these control mechanisms are compromised.
Disease may be caused by a living organism such as viruses, bacteria, protozoa, or fungi. Infection with such organisms can be spread to others by person-to-person contact, or through contaminated water, food, or air. Disease may also be the result of an unavoidable side effect of a medical treatment, or it may be caused by an external substance such as a toxic metal or radioactive substance, in which case it is considered noncommunicable disease.
Once a disease process has been initiated, it usually progresses rapidly, although some diseases such as the acute appendicitis can be quite slow to develop. A disease is characterized as chronic when it begins slowly and persists over an extended period, such as the inflammatory bowel disease ulcerative colitis. Some diseases may recur, in which case they are known as relapsing or remitting.
A disease is categorized as a sign or symptom when it can be identified by physicians, nurses, or family members. Each disease entity has a constellation of signs that is distinctively its own, but some general signs such as fever are shared by many diseases. In addition, a disease can be classified by the increase or decrease in the number of phagocytic white blood cells in the blood (leukocytosis), which is a sign of a heightened immune response to an infection.
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