Comorbidity is a medical term derived from the root word "morbidity." According to the Oxford English Dictionary, morbidity refers to "the quality or condition of being diseased or ill; a pathological state or symptom; a morbid characteristic or idea" as well as "prevalence of disease; the extent or degree of prevalence of disease in a district." In reference to child development, comorbidity is a term used when a child is identified to have two or more simultaneous mental health or physical disorders or diseases. The study of comorbidity is the study of conditions that tend to occur together. For example, there is a high comorbidity of attention deficit disorder and learning disabilities in children, meaning that the prevalence of learning disabilities is significantly higher than expected by chance among those with attention deficit disorder, and vice versa.
Gabbard, G. O. Treatment of Psychiatric Disorders, 3rd edition. Washington, DC: American Psychiatric Association, 2001.