Go get some of that "Simply Orange" orange juice, and if you can tolerate it, the higher pulp content the better.
Vitamin C is a natural boost to the immune system, and natural Vitamin C is as best as you can get. Be weary of any orange juice that's "from concentrate" as the process degrades the vitamin C.
Anytime I feel I'm getting sick, or a family member is getting sick, I down about two of those half gallon jugs.