Why am I always sick?

Last January (2009), I had a tonsillectomy because I kept getting peritonsillar abscesses. Ever since then, I am sick often. I've gotten swine flu, strep throat, cold after cold, UTIs, rashes, fevers, stomach flu, etc. I smoke and drink VERY rarely (maybe twice a month) and I am in good shape. I take care to drink water and eat well, so there is no reason for me to be sick so often! And these illnesses knock me out for several days at a time, causing me to miss school and work. What is going on?