Probiotics

http://en.wikipedia.org/wiki/Probiotic

Advances in Experimental Medicine and Biology, vol.635 - GI Microbiota and Regulation of the Immune System (Springer, 2008).pdf

Probiotic trials:

http://www.sciencedaily.com/releases/2010/05/100519081329.htm

Oral Bacteria May Offer Probiotic Potential Against Upper Respiratory Infections

http://www.sciencedaily.com/releases/2010/06/100618142511.htm