allergies, pollution, exposure to toxins or mold, systemic yeast infections or other fungal
infections, certain bacterial
infections (like Bartonella), Celiac Disease,
and some chronic conditions, like Chronic Fatigue/Myalgic Encephalomyelitis, Fibromyalgia, and a variety of autoimmune disorders.
I have it as part of my Lyme Disease and Bartonella so I know how frustrating it is.