My guess is some mental health disorders, or their severity, are also symptoms of something physiological (from vit D deficiency, thyroid problems, to whatever else). Another guess was, people with mental health disorders experience more stress, maybe they have more cortisol in their blood, which might also make vulnerable. And depression for example also is linked to a heightened white blood cell count in some patients. I think there's more to 'mental' health than we know of today.