Despite a vigorous search, scientists have not yet identified what causes CFS. While a single cause for CFS may yet be identified, another possibility is that CFS has multiple causes. Conditions that have been studied to determine if they cause or trigger the development of CFS include infections, immune disorders, stress, trauma, and toxins.

Infection

Various types of infections have been studied to determine if they might cause or trigger CFS:

  • Epstein-Barr virus infection, also known as mononucleosis
  • Human herpesvirus 6 infection, a virus that can cause problems for people with impaired immune systems, such as AIDS patients or organ transplant recipients taking immune-suppressant drugs
  • Enterovirus infection, a type of virus that enters through the gastrointestinal track and can have no symptoms, mild flu-like symptoms, or rarely severe and even deadly symptoms
  • Rubella, a viral infection also known as German measles
  • Candida albicans, a fungus that causes yeast infections
  • Bornaviruses, which cause borna disease, an infectious neurological syndrome
  • Mycoplasma, a cause of atypical pneumonia
  • Ross River virus, which causes Ross River Fever, a mosquito-borne tropical disease
  • Coxiella burnetti, the agent that causes Q fever
  • Human retrovirus infection, such as HIV, the virus that causes AIDS, or xenotropic murine leukemia virus-related virus (XMRV), a gammaretrovirus

The cause of CFS is unknown, but the condition may be related to infection with effects on the immune system. Several viruses have been studied as possible causes of CFS, but no cause-and-effect relationship has been discovered.