COMORBIDITY In medicine comorbidity is the presence of one or more additional diseases co-occuring with a primary disease or disorder. The additional disorder may also be a behavior or mental disorder. COMORBID A disease or condition The extent to which two pathological conditions occur together in a given population. Two or more coexisting medical conditions or disease processes that are additional diagnosis. There is a high rate of comorbidity between ADHD and other impulse control disorder.