A disease is an identifiable condition affecting an organ or organism whereas illness is the subjective negative experience that comes with ill health. Specifically, in this case, illness is a product of disease although one can be made ill through psychosomatic processes without a disease.
In 1978, Eric Cassell established the distinction between disease and illness by saying "disease … is something an organ has; illness is something a man has." What is meant in this case is that illness refers to feelings that are part of an individual's subjective experience. Illness is typically what sends individuals to the doctor. When the illness, negative feelings and symptoms, abate one considers treatment a success. This incongruity between disease and illness is why doctors advise their patients to complete their fully prescribed dosage of medicines, such as antibiotics. Because illness can disappear before disease, since illness is contingent on the subject experience, one must be vigilant when treating their disease.
The danger of having a disease without an illness, experiencing no symptoms, makes regular check-ups essential for all individuals. The idea of one having an illness without a disease, such as through a psychosomatic process, is a matter of some dispute. Many medical professionals argue that the experiencing of illness, symptoms without the corresponding disease, means that the patient is experiencing a neuro-biological disease that is not yet understood by modern medicine.