Contagious redirects here. For the Isley Brothers song of that name, see Contagious (song).

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

Pathology is the study of diseases. The subject of systematic classification of diseases is referred to as nosology. The broader body of knowledge about human diseases and their treatments is medicine. Many similar (and a few of the same) conditions or processes can affect animals (wild or domestic). The study of diseases affecting animals is veterinary medicine. Plants as well can suffer from a variety of processes such as infection, nutrient deficiency, or deleterious mutation. The study of diseases affecting plants is termed plant pathology.