Mental illness is an illness that affects or is manifested in a person's brain.
It may impact on the way a person thinks, behaves, and interacts with other people. Many people suffering from mental illness may not look as though they are ill or that something is wrong, while others may appear to be confused, agitated, or withdrawn. In the past 20 years especially, psychiatric research has made great strides in the precise diagnosis and successful treatment of many mental illnesses. Recognized mental illnesses are described and categorized in the book Diagnostic and Statistical Manual of Mental Disorders, Fourth Edition. This book is compiled by the American Psychiatric Association and updated periodically.