Sick of Getting Sick

I'm sick of getting sick.
Year 2008 was not a good year for me. I have been sick on and off. Getting diarrhea, gastric flu, flu, fever, cough, cold, backaches, headaches. etc etc etc.
Just so sick of getting sick.
Why do I get sick so easily?
Was it because I don't have enough of exercise?
Was it because the stress that I'm having right now?
Was it because of my research work, that I used to sit in the lab for hours and hours without moving around?
Was it what has happened to me lately, my mood was down?
Or was it because I was getting older and older?
Seems like my energy was not as hyper as last time.
Maybe it's a sign of old age. Going to hit a quarter century life. Gosh! I'm so old.