When I took a medical history from one of my patients he told me, "I belong to the 4H club: hepatitis, HIV, herpes and hemorrhoids."
In medicine you see the term "comorbidity" used quite a bit. It basically just means that a patient has more than one medical problem happening all at once. It isn't specific to any particular combination of illnesses. In forensic psychiatry it usually means mental illness combined with substance abuse, combined with personality disorders. In the correctional world you can add a few extra layers of pathology by throwing in the medical diseases: hepatitis, HIV, head trauma, diabetes and other stuff, like the 4H list given to me today by one of my patients. (On the positive side, he had no history of closed head trauma.)
Practically speaking, what this means for treatment is that everything is going to be a little more complicated. You have to think about how the personality disorder will color the patient's reaction to your care, how the head trauma will affect his ability to understand what you say to him, and what the co-existing medical conditions will do to your choice of psychopharmacology. That can be a challenge. (OK, so the hemorrhoids in today's patient didn't really complicate the pharmacology. At least not until they invent rectal psychotropics.)
Working in a correctional environment actually helps when you're dealing with some of these multiply co-morbid cases. The structured environment gives some predictability and stability to their lives. It takes away some degree of stress in that they don't have to think about where their next meal is coming from. The clear rules and expectations set boundaries for containing the maladaptive behaviors. And while drugs and alcohol certainly do exist in jails and prisons, there's a lower likelihood that the patient will be using inside the walls than in free society. Finally, the patient has access to medical care that he might not otherwise have in the streets so the co-existing medical conditions are less likely to hinder treatment. My job would be much harder if I were treating these folks in free society.
Then again, in free society I'd have a desk and a telephone. And modern ventilation. And office supplies. And an office. Clerical support. A fax machine. Ample parking space. Unlocked restrooms. A vermin-free place to eat. And...
Oh, never mind.