A condition that exists at the same time as another problem or diagnosis. It may or may not be directly or indirectly related to the Chief Complaint or “problem at hand”. So, for example, osteoarthritis or rheumatoid arthritis could be a comorbidity to consider along with a neck strain from using too big a pillow. Blood Thinning medications might reveal a comorbidity that might exist along with headaches and neck pain. The chief complaint (presenting problem) might be influenced by the comorbidity, or the comorbidity might influence the selection of treatment. It is for this reason that a thorough and detailed history be taken, on all initial encounters, and an update history be taken on all subsequent visits, before treatment is rendered.