What is mental illness The World Health Organisation (WHO) defines mental disorders as comprising a broad range of problems, with different symptoms. However, they are generally characterized by some combination of abnormal thoughts, emotions, behaviour and relationships with others. Mental illness can affect people from all walks of life. It cuts across age, class, religion, gender and profession Mental illness can fluctuate, with some people well able to manage their lives, others less so and may need to be hospitalised The first symptoms often develop in early adulthood, but may remain undiagnosed for a long time People refered to mental health services often do not have a definitive diagnosis How a mental illness begins varies from person to person. For some, the illness starts suddenly: the (usually) young person becomes unwell very quickly and quite severely. His or her thoughts may become muddled or he or she may experience hallucinations.For others, the change is gradual and the person may show signs of withdrawal or neglecting themselves. These changes in behaviour can be very difficult to understand especially when no one has recognised that the person is ill.