Baker's yeast may hold some clues per Dr Asim Dasgupta Does Ordinary Baker's Yeast Hold Secret to Curing Common Cold November 22, 1994 New York Times News Service - Sandra Blakeslee editorial.
+ Nutritional Yeast (better/best) @
Beta glucan fiber in nutritional yeast may improve immune function, but there is a concern about lead contamination in some brands. (Check country of origin.) 3/25/2015

+ - "Injecting mice with simple baker's yeast protects against the fatal fungal infection, aspergillosis, according to new research.
The work could lead to the development of a human vaccine that protects immuno-compromised people against a range of life-threatening fungal infections, for which current therapy often fails."