How could my spouse's mental illness affect our marriage?

Marriage can be difficult for a person with mental illness, especially if the spouse has no understanding of the condition. For many people, even with understanding, living with someone who has serious mental illness (SMI) is difficult and stressful. For many, living with a spouse who is mentally ill becomes too much for their own mental health and in many cases separation occurs which may put the unwell person in an unstable and "at risk" position.