A great example is Parkinson's Disease and Pneumonia. Parkinson's really doesn't kill. But, people with Parkinson's often have trouble swallowing and end up inhaling something they shouldn't and/or have trouble coughing and can't get excess mucus out of their lungs. In either case, Pneumonia, which is indeed a disease all to itself and technically has a cause that is separate from the Parkinson's, ends up as a secondary disease. The reason it is secondary is that the weakened condition and specific failings of the patient's body are what create the opportunity for the disease to take hold.
In addition to the secondary health issues, with many diseases and disorders, we see life effects. In other word, something about the disease or disorder causes people to change something about their lives or encourages certain types of emotional or mental stress.
Anybody facing a debilitating condition is likely to change their work and leisure habits. Some go from being vibrant active outdoor types to housebound wallflowers that basically whither away. Caring for and spending time with loved ones, especially children changes. Some disabilities are very visible and even odd looking which creates challenges in social situations. But some disabilities are linked with higher frequencies to certain other issues.
Stiff Person Syndrome is one of these. Presumably, because of the type of problems people with SPS deal with, anxiety is a frequent problem for patients. Many people with SPS have relatively sudden attacks or episodes. Frequently this is caused by a sound or other stimulus. Many patients have the characteristic attacks that give SPS its name. They will suddenly go into some strange pose and be frozen there, even as it causes them to fall to the ground. So, doctors expect anxiety to be a problem. And many doctors still believe that the symptoms of SPS are really psychogenic. Meaning, they are caused by an emotional issue, like anxiety.
But, there is another possible link. Some anxiety medications are linked to the creation of similar chemicals as those that are believed to be lacking in patients with SPS. So, it only seems likely that there is a chemical relationship between SPS and anxiety. In this case, the anxiety is related to if not caused by the disease and is not the cause of the disease.
I know my body and my emotions. I don't believe that I am particularly anxious. And I certainly don't believe that anxiety is causing my symptoms, not even a little bit. But, with the likely chemical connection, it's time I got the insurance company to pay for the anti-anxiety medications the doctor wants me to try.