In medicine, comorbidity is the presence of one or more additional conditions co-occurring with (that is, concomitant or concurrent with) a primary condition; in the countable sense of the term, a comorbidity (plural comorbidities) is each additional condition. The additional condition may also be a behavioral or mental disorder. In medicine, comorbidity is the presence of one or more additional conditions co-occurring with (that is, concomitant or concurrent with) a primary condition; in the countable sense of the term, a comorbidity (plural comorbidities) is each additional condition. The additional condition may also be a behavioral or mental disorder.