Disease is any abnormal condition, affecting all or part of an organism. While most people are familiar with human diseases, it should be noted that other animals, as well as plants, are subject to disease. Dutch Elm Disease, for example, affects trees. When discussing human beings, the term disease is used to refer to any condition that causes some combination of pain, dysfunction, social problems, or death. Disease can be spread by pathogens or occur as the result of a person’s lifestyle.
There are four main types of disease: deficiency, pathogenic, hereditary, and physiological. Diseases can be either communicable or non-communicable. Influenza and AIDS/HIV are communicable diseases. Heart disease is non-communicable. Diseases can be mental or organic. Mental diseases are a broad category that includes major depression, schizophrenia, and anxiety disorders. Organic diseases, in contrast, are those that are caused by a physical or physiological change in the body.
An acute disease is a short-lived disease, such as the common cold. A chronic disease is one that persists for at least six months. Chronic diseases can be stable or progressive, or may go into remission. The term flare-up is used to describe the reoccurrence of symptoms. A refractory disease is one that resists treatment. Progressive diseases have a tendency to worsen over time, eventually resulting in organ failure or death. Epidemiology is the study of the causes of disease.