Have you been to a dicot who diagnosed you? By "glandular fever" are you referring to mononucleosis (http://en.wikipedia.org/wiki/Infectious_mononucleosis)? I use to live in the UK and seem to recall people calling what i've always known as mononucleosis as glandular fever, I never heard the expression before then...
Anyway I had "mononucleosis" in college and was actively exercising before and during according to my doctors advice. "Mono" is a viral disease caused by a virus not exercise. The wikipedia entry I linked to describes it somewhat. This is just my layperson paraphrase of what my doctor told me when I had "mono" aka "glandular fever". It is caused by a virus and is passed along as a virus--one of my friends caught it from me and sadly he ended up dropping out of college as it effected him more! You wouldn't get it from exercise unless maybe you were already exposed to the virus (it does seem to have a fairly long incubation period). I was not ordered to stop exercise after I was diagnosed by a lab test. I was told to avoid contact sports and other activities that might risk rupturing the spleen (a swollen spleen is a symptom). I was in a swimming PE class, and my doctors note just said I could not dive and was to rest if I needed and stay home if "too tired". My friend, on the other hand, was put on bed rest. We actually both went to the same doctor (student health center) and the advice was so different based on our different reactions to the virus. So it can vary how much it effects you so it is always a good idea to consult a doctor about your specifics.
I think some of the symptoms "over training" can cause can feel like some of the symtoms from mono (fatigue, achy joints, lack of mental focus--I had all three of those when I had mono) even in people who were not exposed to the virus. I have had overtraining symptoms myself when I was over zealous about exercise, I can't say whether ti was a relapse of my mono or just related to the training. I tend to think it was training related because in my case a rest week would clear it up. During a rest week--I don't mean "bed rest" like my poor friend had to endure. I mean I would not do vigorous cardio instead I would do fun, moderate, stress relieving activities. Things like walking, yoga, tai chi, mild dance, leisurely cycling, golf--whatever you like that doesn't make you gasp for air (within what you body is comfortable with at that time). And I would eat healthy and prioritize sleep and drinking enough water. After such a week, I usually feel quite revived and find my fitness didn't suffer as I am even stronger and better at vigorous activity than before the rest week. I also find I need to take "rest days" each week. Rest days can include the same activities as I mentioned for my rest weeks. I think it is a little individual how much rest someone needs though. This is just my personal experience though. If you are having a relapse it would probably be best to see your doctor. As far as I know, I never relapsed.
I actually try to have a recovery week every month and 2 rest days a week most weeks. When I get excited about exercise I keep wanting to add more and more so I really have a tendency to overdo it. I'll be fine for a few weeks then crash. If I follow the pattern I just mentioned, the crash doesn't happen unless I get sick. There is a really good article on Spark about overtraining that lists the common symptoms and some tips to avoid it.
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