When I was growing up, yeast was something used to raise bread dough. I was taught to believe it was a basic necessity of life and certainly nothing to fear. But that was then. Now there is a variety of yeast, candida albicans, which can actually invade the body and do serious harm to every organ and system therein.
Modern medicine recognizes that yeast does exist naturally in the body.
Unfortunately, yeast can migrate to the alimentary canal and begin to proliferate there as well. Still this will not create a major problem other than some bloating and constipation and perhaps a little irritation in the lining of the gastrointestinal tract. When population levels of yeast and fungi reach a critical mass, there is an interesting thing that happens. It actually morphs and changes into something other than single yeast cells. This creates a bridge into the rest of the body.