The key to a robust immune system is the gut, If you have a healthy gut then it makes the job for the immune system easier when they need to fight off illness. I would use a quality probiotic, preferably one that isn't synthetic and is human-strain bacteria (as opposed to bovine strain which will only remain in the gut as long as you are taking it) and use daily. Also combine that with healthy and nutricious foods with minimal processed foods. Progurt is one brand to look for.