According to NIMH:
Mental Disorders in America
Mental disorders are common in the United States and internationally. An estimated 26.2 percent of Americans ages 18 and older — about one in four adults — suffer from a diagnosable mental disorder in a given year.1 When applied to the 2004 U.S. Census residential population estimate for ages 18 and older, this figure translates to 57.7 million people.2 Even though mental disorders are widespread in the population, the main burden of illness is concentrated in a much smaller proportion — about 6 percent, or 1 in 17 — who suffer from a serious mental illness.1 In addition, mental disorders are the leading cause of disability in the U.S. and Canada.3 Many people suffer from more than one mental disorder at a given time. Nearly half (45 percent) of those with any mental disorder meet criteria for 2 or more disorders, with severity strongly related to comorbidity.1
In the U.S., mental disorders are diagnosed based on the Diagnostic and Statistical Manual of Mental Disorders, fourth edition (DSM-IV).
That information is copied directly from the www.nimh.nih.gov website.