not an expert on this, since I nearly never get sick. but there are times ones in a while when my throat might feel a little more soar than normal or I'll have a stuffed nose. normally hitting some really micro-nutrient rich foods does the job. my remedy is a big glass of green tea (very antioxidant rich) and a 5-6 oranges (for vitamin C). hits like a truck and doesn't leave any symptoms for survivors.
that's just what works for me though, try it for yourself and see.
I really think though if you have good variety of fresh raw plant foods, you shouldn't be getting sick, at least not for long. also having a regular exercise routine really slims down the chances of having any sickness (which I do utilize as well)
one more thing, do you use cronometer.com? it does a great job of tracking your nutrition, making sure you get enough of certain macro and micro-nutrients and enough energy (aka carbs) according to you gender, weight and activity level
hope you get better))