Recurring respiratory infections!?

During the past 8 or 9 months, I have been getting many respiratory infections. The thing that puzzles me is they are never the same. Since September, I have gone from having the flu, to a regular cold, to strep throat, to another cold, to some bacterial infection (they said something like bronchitis) to, once again now, some type of cold. This has worried me because there have been so many illnesses and they come one right after another. Any thoughts?