My SIL was sick constantly during the end of her teaching degree she started taking fresh parsley from her garden and now she's a full time teacher surrounded by kids being sick all the time and hasn't had much more than a cold. She makes her boys tak parsley now when they're sick and they recover much quicker.
Another trick I heard was, if you couldn't eat garlic was to crush a few cloves into some very warm water and to soak your feet in it for about 20mins to 30mins. The garlic goes straight into your blood stream. My Husband says he feels it working after 20 mins. It definatly lessens the severity of colds and flus
I would also look into getting a food intollerence test done. This is the Australia web site but it is all done from in Germany.
http://www.imupro.com.au/ I'm sure there would be one for the US or what ever country your in. Its the only blood test in the world for food intollerences and very acurate. I used to get sick constantly and even had problems conceiving because my immune system was fighting the food I was eating and not the bad bugs. I found out I had an intollerence to 69 different foods. Its hard to cut out because i basically couldn't eat any processed foods, but as soon as I cut those foods out of my diet within a matter of weeks I had so much more energy, my brain fog lifted, my mood was so much better. (Some of the foods were the main culprates like gluten, wheat, sugar, milk and eggs). The best news was after two months of being on the new diet, I fell pregnant and have been able to continue that pregnancy (I had a previous miscarriage about 12 months before).