Questions about yeast

[Judi Besharah]

My husband and I were pondering the source of yeast the other day - where does it come from?  I 
have read that it is naturally occurring - how?  How is it cultivated?  How is the yeast 
that is used, for instance, in baking, cultivated?  Is is a renewable resource?

Please respond to besharah at ottawa.net

Thank you - I realize this is a very basic question for this newsgroup, and appreciate your 
time.

Judi Besharah