Unlike bacteria, Yeast is single-celled microorganisms with a nucleus enclosed within membranes. Consequently, yeast is classified as a member of the fungus kingdom. The first yeast originated hundreds of millions of years ago. Compared to bacteria, 1,500+ species of yeast have been identified to date. Similarly to bacteria, yeast comprise some of the natural microbial flora living in, and upon, animals and huumans. Huumans have been using yeast in baking and brewing for centuries, but have suffered from its scourges for much longer due to Candida albicans, the most notorious of the yeast inhabitants in the huuman body (see: Wikipedia).