Mental illness vs Mental disorder what is the difference?

The terms can be used interchangeably. If I am not mistaken, mental health practitioners refers to most (or all) conditions as "disorders", following how they are being listed in American Psychology Association (APA) Diagnostic and Statistical Manual of Mental Health Disorders. Personally, I feel that "mental illness" carries a negative connotation while "mental disorder" is a more neutral term. But this is just me.