Psychologist here, depends on what you mean by treated. Most mental illnesses aren’t like a cold where you’re able to take some medication and get rid of it, they’re more like a chronic back injury that you learn to manage. For most people, some combination of therapy and chemical treatment is sufficient to allow them to live a life where their mental health is managed. There are people whom chemical treatment doesn’t work on, sometimes because of unhealthy brain chemistry, and who are unwilling or unable to participate in therapy. Unfortunately for these people, there’s not much that can be done for them short of a miracle.