The Diagnostics Statistic Manual of Mental disorders (now in its 5th edition, and known as DSM-V) has the corporate authorship of the American Psychiatric Association. This manual lists every known mental "disorder" known to the APA and lists the specific criteria that will help clinicians assess and diagnose clients as objectively and accurately as possible.
The DSM-V ascribes these conditions primarily as "disorders". The distinction often found in the source of the condition; if the source is physiological the language changes and the word "illness" takes the place of "disorder". Other than that, they are used interchangeably.
Even the Mayo Clinic combines both terms to define "mental illness"
Mental illness refers to a wide range of mental health conditions — disorders that affect your mood, thinking and behavior.
However, from the perspective of the DSM-V, a "disorder" refers to any type of abnormal activity that takes place in the human brain and which renders it unable to perform as it should. The Merriam Webster dictionary defines the word "disorder" as "a disturbance in normal function".
An "illness", on the other hand, is a disorder that is caused specifically by a physiological, or biological source and which results in disease. More specifically, an illness entails the use of invasive and more aggressive procedures to counteract its systemic effects. In some cases, it is an underlying illness that causes classical mental disorders.
Merriam Webster defines "illness" as an "unhealthy condition".
An example of mental disorders of high incidence would be
- bulimia- or the habit of binging on food and then throwing it up for fear of gaining weight
- anxiety- a generalized state of heightened worry and nervousness
- phobia- or extreme terror of something (with or without foundation)
- maniac behavior- or extreme mood swings that may or may not have a physiological basis.
Examples of conditions that are considered "illnesses" based on their specific pathology include:
- AIDS-related dementia
- Delirium Tremens- or severe alcohol withdrawal affecting the whole nervous system
- Post-partum psychosis
- Acute and Transient psychotic disorder (ATPD)
Again, there is not a clear description that could make it easier to extrapolate traits unique to "illness" versus "disorder". It is mainly in the manner in which the language is used regarding the origin and intensity of the condition where a slight difference in their meaning can be delineated.