Disease is a term for any condition that impairs the normal functioning of an organism or body. Although plants and animals also contract diseases, by far the most significant disease-related areas of interest are those conditions that afflict human beings. They can be divided into three categories: intrinsic, or coming from within the body; extrinsic, or emerging from outside it; and of unknown origin. Until the twentieth century brought changes in the living standards and health care of industrialized societies, extrinsic diseases were the greater threat; today, however, diseases of intrinsic origin are much more familiar. Among them are stress-related diseases, autoimmune disorders, cancers, hereditary diseases, glandular conditions, and conditions resulting from malnutrition. There are also illnesses, such as Alzheimer's disease, whose causes remain essentially unknown.