This is a good question. When talking about mental health, a mental disorder and mental illness are very similar, if not the same.
Sometimes a mental illness is considered something that is more biologically based, and more long standing. However, in truth a mental disorder (dis-order, disorganization) and a mental illness are the same.
Why the two different terms? I think it has more to do with the "camp" the professional discussing the problem is in. A medical doctor/psychiatrist may be more inclined to use the term "illness" while a professional counselor or psychotherapist may be more inclines to use the term "disorder."
Again, good question!
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