There are so many different variants of the flu/cold that you can just keep getting em over and over..
BUT: here's a decent way to boost your immunity.
Like someone else mentioned, Activia yogurt helps immunity...
Also, a good HID light (Used for seasonal affective disorder) can help as can nasal irrigation.. just get a BUNCH of bottles of nasal saline, and use it 6-8x a day as directed on the bottle...
this with vitamin C, Zinc, B-Complex and echinacea (sp?) can help too... do NOT take all the vitamin C at one time! take a dose 3-4x a day... it is water soluble and anything not "used" gets wasted. Take vitamins with food... it helps absorption.
Eat a LOT of spicy foods-- it has been proven to shorten the legnth of colds and flu as well as boost the immunity... some chili's or cayenne in home-made broth works wonders!
If you aren't a spicy-food-fan, go with capsacin capsules-- capsi-cool is a good one that won't hurt your tummy-- and isn't real expensive!
Good luck!!!