Disease is an illness or abnormal condition that negatively affects the structure and/or function of part or all of the body and is associated with specific signs and symptoms. It is the result of a disruption of the delicate physiological balance (homeostasis) among chemical, physical and functional processes.
The term “disease” encompasses a wide range of illnesses and disorders, from mild to severe. It also can be applied to conditions that appear for only a short period of time, whereas others may last for the entire length of one’s life. Infectious diseases are caused by microorganisms such as bacteria, viruses, fungi or parasites, and are spread from person to person by direct contact, through infected animals, or via the environment (water, food, etc). Non-infectious diseases are generally acquired during one’s lifetime (acquired disease), whereas congenital diseases were present at birth (congenital disease).
An important distinction between disease and normal health is that when a disease develops, it results from an imbalance between the organism’s defence mechanisms and the causative agent. Many factors, including resistance, nutritional state, age and immunological status of the host, as well as the virulence or toxicity of the agent, play an important role in whether a disease will develop.
As medical science advances, it is sometimes possible to identify the cause of some diseases; however, it is quite common for a disease to have unknown causes, and thus the label idiopathic disease remains. This can apply to syndromes such as Down Syndrome (a consequence of an extra chromosome at birth) or to diseases such as Parkinson’s disease.
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