Hi CarebearL,
There is no doubt that stress wreaks havoc with the immune system, increasing the risk of infection, autoimmune disorders, stress related disorders and other serious diseases. As you read, you will find a lot of references to "self-care". It is critical that in addition to therapy, that someone with PTSD focus on the basics of eating right, getting plenty of rest, exercise, hydration, etc.
Our minds and bodies are interconnected in ways that science is only beginning to tap into. A holistic approach to battling this disorder is the best route to take in my personal opinion. Being your best physically allows a person to have the energy and focus that it take to get better overall.