Disease and illness are terms that are often used interchangeably by physicians and the lay public. But not all usage permits this. For instance, diseases are referred to in terms of entities with etiologies; illnesses are not. We also speak of illness as being the effect or symptom of a disease, but not the converse. In what follows, disease and illness will be treated as distinct concepts.I thank Dr. Rosamond Rhodes, Director of Bioethics Education, Mount Sinai School of Medicine, for her extremely helpful questions and comments concerning this article.