[I]ndividuals with one mental disorder are at a high risk for also having a second one (comorbidity). Nearly half (45 percent) of those with one mental disorder met criteria for two or more disorders, with severity strongly related to comorbidity. This finding supports the suggestion by a growing portion of researchers that the boundaries between some diagnostic categories may be less discrete than previously believed.
From NIMH in an executive summary of a 2005 report. An abstract of 2007 survey fleshes this out further using a very good data set:
The population prevalence of DSM-IV personality disorders (PDs) remains largely unknown. Data are reported here on the prevalence and correlates of clinician-diagnosed Clusters A, B, and C DSM-IV PDs in the general population of the United States. . . . prevalence estimates were 5.7% Cluster A, 1.5% Cluster B, 6.0% Cluster C, and 9.1% any PD. All three PD clusters were significantly comorbid with a wide range of DSM-IV Axis I disorders. Significant associations of PDs with functional impairment were largely accounted for by Axis I comorbidity. CONCLUSIONS: Strong Axis I comorbidity raises questions about the somewhat arbitrary separation of PDs from Axis I disorders in the DSM nomenclature. The impairment findings suggest that the main public health significance of PDs lies in their effects on Axis I disorders rather than in their effects on functioning.
I'll try to circle back to define the DSM-IV terminology in this abstract later. More articles relying on the same data can be found here. Also particularly notable is a quite readable and up to date prevalence chart for a wide range of mental health conditions.