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In medicine the term comorbidity is used to describe diseases and conditions that often occur together with another disease or condition. The first disease or disorder is called primary. In many cases, the primary condition or disease does not cause the comorbid disorders. However, they may both be caused or partly caused by the same thing, for example a genetic mutation or a traumatic experience.

As an example, a patient may have antisocial personality disorder and attention-deficit hyperactivity disorder. The two conditions are comorbid. Another patient might suffer from borderline personality disorder, posttraumatic stress disorder and bipolar disorder. The three conditions are comorbid.