A severely compromised Immune System?

That is, my immune system. My friends and family have known this for a while now - the frequency with which I seem to fall ill - all the common illnesses strike me quite commonly, so to speak - be it cold or fever or influenza. In fact, currently I am in the middle of recovering from a debilitating influenza attack. I got hit by the virus some time last Tuesday when I visited a cardiologist in the Gleneagles hospital here (that's another story altogether, but so far nothing seems to be wrong with my heart, so no worries there). Since then, I have been on medical leave for 7 continuous days and I still have not recovered. Days 3 to 5 were especially bad when I was sneezing uncontrollably and feeling weak and dizzy all the time. Today was the first time in a week that I went to work. The phlegm is still there in my system and my nose is still blocked. At the end of the day, I started sneezing once again just as I was leaving work. And in between all this, I have been unable to sleep well in the night for some odd reason (despite the supposedly "knock-out" medicines).

So what's wrong with me? I know it has to be my immune system. I maybe termed obese in the strictest numerical sense, but I do not think that that is the cause of all this - I do not experience the symptoms of obesity and I quite love to (and without any difficulty as well) run and indulge in physical activities. So what happened to my immune system? And when? I do not recall being susceptible to these common illnesses during my 4 years in the US. And before that I do not recall falling sick at all when I did my Junior College in Singapore. It's only after coming back in 2004 that my immune system has tanked. I do agree that unhappiness at work is a contributing factor. But that much? I am not that unhappy at work (at least not yet) that it would impact my immune system so severely (although I attribute my hypertension problem solely to the torturous and agonizing first few months at work). So what happened? Do you have a clue? And more importantly, do you know someone (a doctor or wise old men/women or a body of knowledge) who can suggest ways on how to rebuild my immunity (preferably without ingesting big pharma tablets into my body)? I would be really grateful if someone could point me to such a source. Because I think the time has come for me to really address this issue...I will be 30 in a couple of years and I want to be in good health before I hit that number.

Yes, I am a bit scared, somewhat in despair and quite lost as to what to do.