In some cases,
chronic fatigue syndrome (CFS) develops after a flu-like
illness such as
mononucleosis (mono) or after a period of unusual stress. But it may
also occur without warning, even if you have not been sick.
fatigue may come upon you gradually or quite suddenly. Because fatigue can be
vague and can be caused by many things, you might not pay attention to the
problem for several weeks or months. It is hard to say what is normal with CFS. That's
because the diagnosis often is not clear for some time.