Here is a leaflet from Inclusion Europe and Mental Health Europe:
The Differences between Mental Illness and Intellectual Disability
Intellectual disability and mental illness are often confused. However, they are very different! Only if you know about the differences, both groups can be treated adequately. Mental illness is an illness and can be cured, whereas intellectual disability is a life-long condition. This leaflet sets out to explain the most important differences between intellectual disability and mental illness.
People with mental illness need mental health care, medical and psycho- therapies. In general they have no intellectual impairment and can live independently when their mental health problem is controlled.
People with intellectual disability are not ill, unless they have a common illness. They need easy-to-understand information, education or training to live included in society.
Treating the two different groups as one leads to wrong assumptions, faulty service planning, and often to discrimination of one group by the other. It is clear that medical doctors, therapists and education professionals must be able to distinguish the two groups as basis for adequate interventions. But also policy makers at local, national and European level must recognise the differences to be able to provide for both groups the necessary support systems.