Take immune boosters for a while and don't stop taking them.
I take 3 x 1000mg of Vit B complex (from Costco) and a minimum of 3 x 1000 a day of Vit C. When I feel something coming I take 10 x 1000 a day for 3 days and I don't get sick. Don't bother with that cold fx crap. It's over priced. Vit A, Vit E and Zinc are good too.
Oh and see a Doctor!
http://voices.yahoo.com/vitamins-boost-immune-system-1983139.html