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.