Disease





In common usage, a disease is any abnormal condition of the body or mind
that causes discomfort or dysfunction. Often used metaphorically for
pathological conditions of other things, as in disease of society. Stricter
medical usage sometimes distinguishes a disease, which has a known specific
cause or causes (called its etiology), from a syndrome, which is a
collection of symptoms that often occur together but for which there is no
known cause. Also, many medical terms that describe symptoms are often
called "diseases", especially when the cause of the symptom is unknown.

The largest and best-known category, infectious diseases are those caused by
transmissible infectious agents such as bacteria, fungi, parasites, viruses,
and prions. Closely related though not infectious diseases in the strictest
sense are parasitic diseases caused by protozoa and worms. There are also
genetic diseases caused by the presence or absence of genes in the affected
person's DNA; toxic diseases caused by exposure to environmental toxins such
as heavy metals; nutritional diseases caused by lack or deficiency in
certain nutrients; conditions caused by injury, malformation, or disuse of
parts of the body; autoimmune diseases caused by immune system attacks on
the body's own tissue; diseases caused by the patient's own beliefs; and
diseases causes by combinations of these, and of course totally unknown causes.