Certain terms...

Disease : A disease may be defined as an entity with a cluster of characteristic signs / symptoms (clinical picture), having a well defined course and response to treatment, associated with characteristic pathological changes in an organ or organ system of the body. Usually it has identifiable cause and risk factors.

Disorder : This is a group of signs / symptoms occurring together (this is called a syndrome in medical parlance) presumed to be resulting from a particular pathological process. The relationship of this process to the clinical picture is not proved beyond doubt as in the case of disease.

Illness: It is a person's subjective feeling / perception or impression that he is unwell. This may or may not be the result of underlying pathological processes.

Sick role: is the role assigned by the society to a person who is ill. S/He is expected to seek treatment and follow advice of the healing professional. He is also exempted from his usual obligations like work, caring for family etc.

Illness behavior is the behavior exhibited by a person who thinks he is ill. He abstains from work and other responsibilities.