FMS, and especially CFS, have been viewed as disorders of the immune system, with underlying viral or bacterial infections suspected as cofactors. No specific pathogen or immune abnormality has been consistently or uniquely associated with either illness.
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In theories pointing to immune dysregulation, an agent causes an initial infection, dysregulating the immune system and thus allowing previously dormant viruses to emerge from latency and actively reproduce. Cytokines, produced in response to reactivated viruses, continue to cause symptoms. Activation of certain cytokines may cause disruption of the hypothalamic-pituitary-adrenal (HPA) axis, with abnormal levels of HPA-related chemicals creating a vicious cycle and persistent symptoms.
(Source: Chronic Fatigue Syndrome, Fibromyalgia, and Other Invisible Illnesses – The Comprehensive Guide, by Katrina Berne, Ph.D. Published by Hunter House books and available at www.hunterhouse.com.)