The term morbidity refers to the incidence of disease, & comorbidity is the occurrence of two disorders or illnesses in one individual
These disorders coexist when a person experiences the effects of both simultaneously, regardless of which disorder appeared first. The term is commonly used when describing a link between mental illness and drug abuse due to the high rate of occurrence of both disorders among American men and women.
Studies examining both types of disorders reveal that six out of ten people labeled with a substance abuse problem also live with a form of mental illness. The link by researchers does not conclude that mental illness caused the substance abuse or that substance abuse caused the mental illness. While it is possible that one disorder may have caused the other, the same risk factors may have caused both disorders in an individual.
Studies examining the occurrence of mental illness and drug abuse suggest one common risk factor may be genetic predisposition to both conditions. Research also suggests these individuals experience an increased risk of developing the second disorder after the first one appears. Those individuals with a genetic predisposition may also be more vulnerable to environmental factors, such as stress, physical and mental abuse, and diet.