What causes chronic fatigue syndrome or systemic exertional intolerance disease?
No defined cause of CFS or SEID is known, even after about two decades of research on patients who fit the CFS or SEID criteria. Although many diseases coexist with CFS or SEID in patients, there are no proven links to any known disease (physical or mental) or pathogen (including viral) that is responsible for CFS or SEID development.
The Centers for Disease Control (CDC) indicates that researchers are still trying to identify the cause(s) of CFS/SEID and offer some speculation about the ongoing research. For example, they suggest the possibility that CFS/SEID represents an endpoint of multiple diseases or conditions such as viral infections, stress, and toxin exposure. However, the CDC states that "CFS is not caused exclusively by any single recognized infectious disease agent." This includes Epstein-Barr virus, Lyme disease bacteria (Borrelia burgdorferi), human retroviruses, bornaviruses, fungi, Mycoplasma spp, and many others. However, if a person has been infected with several (at least three) different pathogens, the chances of getting CFS/SEID goes up. In addition, some researchers had suggested that a new virus found in some CFS/SEID patients (termed XMRV or xenotropic murine leukemia virus-related virus) may be a candidate for cause, but a recent larger study has disproven this theory. In addition, although the CDC says no autoimmune changes like lupus or other diseases are found in CFS/SEID, many CFS/SEID patients have high levels of immune complexes and anti-self antibodies in their blood that may be a clue about what causes CFS/SEID. The CDC mentions other findings (allergies, T-cell activation, and cytokines), but none have any direct link to causing CFS/SEID.