Mental illness is a general term that refers to a group of illnesses that affect the brain. A mental illness is a health problem that significantly affects how a person feels, thinks, behaves, and interacts with other people. Mental illnesses are diagnosed according to standardized criteria. The symptoms of mental illness may interfere with people’s lives in different ways and to different degrees. The severity of mental disorders can be classified as mild, moderate or severe. This is based on what impact the symptoms have on someone’s home management, social life, ability to work and relationships.