Yes, yes, yes to everything Greg S said! And adding to that...
I used to get sick all the time - every virus my kids had, I also got. A couple of years ago, my doctor recommended that I start taking 5000 IUs of vitamin D per day. In the past 2 1/2 - 3 years, I have only had one cold. And, I have been exposed (through my family) to everything from whooping cough to the influenza! Even the one cold I had, was pretty negligible for me - just a little post nasal drip and runny nose for a day or two - I didn't even realize I was sick and unfortunately passed on to my kids. For them, it was a fever, sore throat, headache, nasal congestion & running for ten days.
You can have your vitamin D levels checked via a blood test from your doctor. Optimal levels are 70-80. Most people in the United States are below 30 - which impairs your immune system.
Also, avoid sugar at all costs. It destroys your immune system. Here's a couple of articles from
Dr. Sears http://www.askdrsears.com/topics/family-nutrition/sugar/harmful-effects-excess-sugar
or do a quick google search on "sugar's affect on the immune system".