Causes. The fungus candida causes a vaginal yeast infection. Your vagina naturally contains a balanced mix of yeast, including candida, and bacteria. Lactobacillus bacteria produce acid, which prevents yeast overgrowth. That balance can be disrupted and lead to a yeast infection.auses. The fungus candida causes a vaginal yeast infection. Your vagina naturally contains a balanced mix of yeast, including candida, and bacteria. Lactobacillus bacteria produce acid, which prevents yeast overgrowth. That balance can be disrupted and lead to a yeast infection.