Our friends write in the Merriam-Webster Medical Dictionary…

dis·ease; Pronunciation: diz-'ez; noun

"an impairment of the normal state of the living animal or plant body or one of its parts that interrupts or modifies the performance of the vital functions and is a response to environmental factors (as malnutrition, industrial hazards, or climate), to specific infective agents (as worms, bacteria, or viruses), to inherent defects of the organism (as genetic anomalies), or to combinations of these."

A disease is any abnormal condition of the body or mind that causes discomfort, dysfunction, or distress to the person affected or those in contact with the person. Sometimes the term is used broadly to include injuries, disabilities, syndromes, symptoms, deviant behaviors, and atypical variations of structure and function, while in other contexts these may be considered distinguishable categories.

Illness can be another term used for disease or it can be a person's preception of their health without regard to having a disease. A person without any disease may feel unhealthy and believe he has an illness. Another person may feel healthy and believe he does not have an illness even though he may have dangerously high blood pressure (hypertension which may lead to a fatal heart attack or stroke.
Pathology is the study of diseases. The subject of systematic classification of diseases is referred to as nosology. The broader body of knowledge about diseases and their treatments is medicine.

Older medical usage sometimes distinguished a disease, which has a known specific cause or causes (called its etiology), from a syndrome, which is a collection of signs and/or symptoms that occur together. This pedantic distinction has become even less valid as the causes of many syndromes have been identified. Also, many medical terms that describe symptoms or abnormalities may be referred to as "diseases" in many contexts, especially when the cause of the problem is unknown.

A condition can be objectively verifiable, but considering it a disease is a social value judgement. For example, in current North American society the number of people considering shortness and obesity as diseases to be treated has been increasing over the last 40 years, and the number of people who consider homosexuality to be a disease has been decreasing.

A condition may be considered a disease in some cultures or eras but not in others. Oppositional-defiant disorder, attention-deficit hyperactivity disorder, and sociopathic personality disorder are examples of conditions considered diseases in current North American society but not recognized in American culture a century ago or in many other cultures currently.

Sometimes whether a condition should be considered a disease or a variation of human structure or function becomes an intensely political controversy because of significant social or economic implications. For example recognition of post-traumatic stress disorder, also known as shell shock, were highly politicized processes in the United States, as was repetitive motion injury in Australia.
One of the largest and best-known categories of disease, 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 caused by combinations of these, and of course totally unknown causes.