<P> Depression and pain often co-occur . One or more pain symptoms are present in 65% of depressed patients, and anywhere from 5 to 85% of patients with pain will be suffering from depression, depending on the setting; there is a lower prevalence in general practice, and higher in specialty clinics . The diagnosis of depression is often delayed or missed, and the outcome can worsen if the depression is noticed but completely misunderstood . </P> <P> Depression is also associated with a 1.5 - to 2-fold increased risk of cardiovascular disease, independent of other known risk factors, and is itself linked directly or indirectly to risk factors such as smoking and obesity . People with major depression are less likely to follow medical recommendations for treating and preventing cardiovascular disorders, which further increases their risk of medical complications . In addition, cardiologists may not recognize underlying depression that complicates a cardiovascular problem under their care . </P> <P> The cause of major depressive disorder is unknown . The biopsychosocial model proposes that biological, psychological, and social factors all play a role in causing depression . The diathesis--stress model specifies that depression results when a preexisting vulnerability, or diathesis, is activated by stressful life events . The preexisting vulnerability can be either genetic, implying an interaction between nature and nurture, or schematic, resulting from views of the world learned in childhood . </P> <P> Childhood abuse, either physical, sexual or psychological, are all risk factors for depression, among other psychiatric issues that co-occur such as anxiety and drug abuse . Childhood trauma also correlates with severity of depression, lack of response to treatment and length of illness . However, some are more susceptible to developing mental illness such as depression after trauma, and various genes have been suggested to control susceptibility . </P>

Common comorbid disorders diagnosed with major depressive disorder include