Mental disorders are common in the United States and internationally. An estimated 26 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 58 million people. 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.
In addition, mental disorders are the leading cause of disability in the U.S. and Canada for ages 15-44. Many people suffer from more than one mental disorder at a given time. Almost half of those with any mental disorder meet criteria for 2 or more disorders, with severity strongly related to comorbidity.
1. Kessler RC, Chiu WT, Demler O, Walters EE. Prevalence, severity, and comorbidity of twelve-month DSM-IV disorders in the National Comorbidity Survey Replication (NCS-R). Archives of General Psychiatry, 2005 Jun;62(6):617-27.
Authors bio is coming up shortly.