Could I have a low immune system for any particular reason?

I just got over being sick with a fever and couch and congested, it last 10 days I have been healthy all of 5 days and now I have a sore throat and feel sick again. This seems to be my normal routine I am sick at least once a month for 10 -12 days. I am pretty healthy eater so I dont understand why I am always sick? any suggestions on what may be the reasoning?