Yeast is a type of fungus.  The type of yeast that typically causes vaginal infections is candida albicans.  Candida albicans often live harmlessly in the body and are kept in check by other protective bacteria.  However, unbalancing substances in the body, such as antibiotics, can kill the protective bacteria, allowing yeast to multiply.  Not kept in check, the yeast can flourish, leading to a vaginal yeast infection.  Watch this video to learn more about the growth of yeast in your body!

More: What Causes Yeast Infections?