It is a common misconception that vegans don't eat yeast; yeast is not of animal origin, so it is vegan-friendly. Yeast is a single-celled organism that feeds on sugar and releases alcohol and carbon dioxide, which creates the air bubbles in bread. It is also used to ferment and flavor foods.
Yeast is a naturally occurring fungus that floats through the air. When making fermented foods, such as sauerkraut or pickles, microscopic yeast lands on the food and aids in the fermentation process. It has been used for at least 5,000 years, since the ancient Egyptians used it as a leavening agent for bread.