How come I get sick so often?

I've posted on here before about this but was looking for new ideas. I have in the past six months been sick five times. Not five colds but seriously sick five times. Bronchitis 3 times, walking pneumonia once, and now a cold ( I hope). I started working full time six months ago and breathe in a lot of dust as well as go to school part time and commute a lot in the bay area. At first i thought the stress was suppressing my immune system so i started taking vitamin c and multivitamins, using hand soap, hand sanitizer, etc. I've been to the doctor a few times now and no answers. When I get sick it's not just a day and then i'm fine, i'm out of work a whole week out of the month with a high fever and cough and other symptoms that prevent me from even getting out of my apt. I thought moving to california from the midwest would help but apparently not. I am seriously sick of being sick and dont know what to do. I've been missing so many days from work and have no sick leave. Help!