why am I always sick?

I have a hx of severe vit d deficiency. It has improved some over the years. I am an er nurse and I work about 60 hours a week. I have no trouble sleeping, in fact I can fall asleep anytime, anywhere. The problem is I am always sick. Whatever is going around I will catch it. I take a mvi, vit d3, and zinc daily. I drink mostly water. I am overweight and recently joined a gym. Trying to workout 3 days a week. Any suggestions for what else I can do to boost my immune system? I do have reynauds. I have been tested for ra and lupus but my results were elevated for both which was blamed on low vitamin d. I have not been retested since vitamin d level has improved.