How do you know you have a mental illness?

This is the question on Quora. Answered by Alana Winner, BGS Psychology and Behavior, University of Kansas: "Some people do not know. Well meaning friends and family might have to let the person know something is not right. If something is going on that interferes significantly with your ability to work, go to school(if you go) and causes you much distress and you think it is related to a mental problem visit your M.D. or a mental health specialists. Sometimes it is hard to know if there is a medical cause for your problem and you might be referred to see your doctor. For example, people with anxiety that say they had a panic attack with chest pain, might be asked to see their doctor to make sure something else is not wrong".