Hi Tara, that is an interesting & very helpful topic. If you don't mind, I'm going to add a short definition of 'Comorbidity' on your post that I found on Wikipedia:
In medicine, comorbidity is the presence of one or more additional diseases or disorders co-occurring with a primary disease or disorder; in the countable sense of the term, a comorbidity is each additional disorder or disease. The additional disorder may, also, be a behavioral or mental disorder.
I like their definition. Examples: primary brain injury with secondary gastrointestinal disorder. The person never had stomach issues a day in their life but the brain injury (anoxia/lack of oxygen) caused the damage to the GI tract secondary to primary process. Or a diabetic who later suffers hypertension secondary to the diabetes (precipitated by the primary diagnosis). I thought examples may help depict.