How does mental health affect a person's physical health?

Axiety: Here is one example. Let's say a person becomes very anxious, for whatever reason. If he/she is anxious enough and for a long enough period of time, sleep loss begins, and then overeating or undereating, then the immune system can slowly be compromised, which means more physical illness, and in dire, extreme prolonged cases, ulcers and cardiac issues, just to name a few.