The cause of the illness is not yet known. Current theories are looking at =
the
possibilities of neuroendocrine dysfunction, viruses, environmental toxins,=
 genetic
predisposition, or a combination of these. For a time it was thought that E=
pstein-Barr
Virus (EBV), the cause of mononucleosis, might cause CFS but recent researc=
h has
discounted this idea. The illness seems to prompt a chronic immune reaction=
 in the
body, however it is not clear that this is in response to any actual infect=
ion -- this may
only be a dysfunction of the immune system itself.

A recent concept promulgated by Prof. Mark Demitrack is that CFS is a gener=
alized
condition which may have any of several causes (in the same way that the co=
ndition
called high blood pressure is not caused by any one single factor). It *is*=
 known that
stressors, physical or emotional, seems to make CFS worse.

Some current research continues to investigate possible viral causes includ=
ing HHV-6,
other herpes viruses, enteroviruses, and retroviruses. Additionally, co-fac=
tors (such as
genetic predisposition, stress, environment, gender, age, and prior illness=
) appear to
play an important role in the development and course of the illness.

Many medical observers have noted that CFS seems often to be "triggered" by=
 some
stressful event, but in all likelihood the condition was latent beforehand.=
 Some people
will appear to get CFS following a viral infection, or a head injury, or su=
rgery,
excessive use of antibiotics, or some other traumatic event. Yet it's unlik=
ely that these
events on their own could be a primary cause.