Unfortunately, when it comes to combat posttraumatic stress disorder (PTSD), additional mental illnesses occurring with combat PTSD is almost the rule, rather than the exception. When one diagnosis exists with another, this is known as comorbidity. Studies have found that of veterans with combat PTSD, about half have an additional, current mental illness diagnosis. Comorbidity makes treating combat PTSD more complicated and, of course, tends to increase suffering for the patient. Here is some more information about mental illnesses that commonly occur with combat PTSD.