Mental disorders (or mental illnesses) are conditions that affect your thinking, feeling, mood, and behavior. They may be occasional or long-lasting (chronic). They can affect your ability to relate to others and function each day.Mental disorders are common. More than half of all Americans will be diagnosed with a mental disorder at some time in their life.Treatment depends on which mental disorder you have and how serious it is. You and your provider will work on a treatment plan just for you. It usually involves some type of therapy. You may also take medicines. Some people also need social support and education on managing their condition.