What is the current view of the medical profession with regard to the
types and causes of 'sub-chronic' fatigue - i.e., fatigue which is not
as severe or long-lasting as Chronic Fatigue Syndrome but nontheless
occurs with a frequency and severity which has a significant negative
effect on one's daily life yet is not obviously explained by lack of
sleep, physical activity, etc.?

I don't know what the medical profession says about heavy metal
toxicity but can you imagine how a body would feel if it was
collecting heavy metals from the food, air and water?  This is a broad
subject.  If you have any silver fillings in your teeth (which are
about 50% mercury and you know how toxic that is) that is a very
direct source of suppression on your system.  You could study up on
the topic for quite awhile. If one wants to get rid of heavy metals
two good sources are Dr. Hal Huggins for "getting metals out of your
mouth" and Dr. Sherry Rogers in her book "Detoxify or Die" for getting
metals out of your body.  I mention heavy metals at the moment but
chemicals in general also interfere with healthful metabolism.

CYTOMEGALOVIRUS. A bug that resides in everyone, mostly with no
adverse effects. But when a person gets run down it can 'jump up and
bite you' . The major effect is like chronic fatigue syndrome, and it
can totally disrupt a persons life. The problem is, most medical
practitioners give it no credence as having any bad effects. Simply
because most people carry it.

If you're not confident that its Chronic fatigue syndrome, I would
start lookin at maybe a chronic anaemia.

The major types of anaemia which cause fatigue/lethargy in patients
are an Iron or B12/Folate Deficiency or form of thalassaemia. The
deficiencies pretty much speak for themselves, a lack of iron,
B12/folate hinder the production of haemoglobin and red cells
respectively. When haemoglobin (carries oxygen around the body) and
red cells decrease in number/concentration your body's ability to
oxygenate your cells becomes lessened and fatigue/lethargy is the
major symptom. In thalassaemia, the patient has a genetic condition
(either alpha or beta) which results in the mal-production of the
haemoglobin chains, which in turn reduces the oxygenation of cells in
the body.

To rule any of these out I need more info about you, mainly:
sex/age/nationality. If your female and have been having regular
periods you could be losing blood and not replacing iron sufficiently
resulting in a deficiency for example.

Also I wouldn't rule out what others have said i.e. heavy metal
poisoning and CMV. However these are unlikely because other symptoms
do arise which you havn't mentioned!

:)