I know that psychiatric illness can disrupt immune system and endocrine function, but so can physical illness.
Just to be the pedant that I am, note that "psychological" is certainly a fair description of say depression, in the abstract, if one sets aside how the word might be mis-perceived. But in practice, everyone mis-perceives it!
Everyone perceives that it means these illnesses have a psychological origin or cause, not merely a psychological nature. But that is not established. I am one who believes it likely that no psychogenic diseases exist, except maybe PTSD. Many groups including some in high places (Harvard) find that elevated cytokines are a likely cause of major depression; however this is hotly disputed.
Of course, in science we assume without overwhelming proof that the psyche is made of neurons and their actions, because there is no way to address the psyche scientifically if this isnt so. I still believe that a fairly clear distinction between psychogenic and physiogenic can be made. The former occurs via experiences, via what comes into the "psyche" and how it is processed. An event in the psyche should precede any physical change that can be summarized in the sense of "a great percentage of dopaminergic neurons in the substantia nigra died". The physical changes that correspond to an event in the psyche presumably lack this simplicity; they should instead contain incompressible information such as "this neuron fired, then this one and this one, and 25 milliseconds later, this one did", etc.
I do think that a pyschogenic disease, if any exist, could cause summarizable, compressible physical events that are causally downstream of the disease cause. For example, depression *could* be psychogenic, and the psyche could, eventually, cause the elevated cortisol found in something like 1/2 of depression patients. The cortisol change could even be upstream of the depression experience (altered qualia and behavior). But only a psychological-neural phenomenon with no simple physical description can be at the top, if it be a psychogenic disease. If a great percentage of dopaminergic neurons in the substantia nigra die, this too could have a psychological cause -- but only if the less-compressible psychological phenomenon happens first.
So yeah, depression-ologists often take depression to be psychogenic and yet to very often show elevated cortisol. Lloyd is doing the same thing they are doing. The only absolute reason to believe that Parkinson's disease is *not* psychogenic is because the correlated physical phenomenon (death of dopaminergin neurons in the substantia nigra) is more spectacular than elevated cortisol.
Actually there is one more reason. Most diseases lower fitness and there should, ideally, be some reason why they occur nevertheless. I think many people find it easier to see how psychogenic depression could happen nevertheless, and cant see why psychogenic Parkinson's could happen nevertheless. I dont really agree, but it would take another page or two to explain.