ANDY'S ANSWER: A yeast infection is caused by a an overgrowth of the naturally-occurring fungus in the body called candida albicans.
An overgrowth of this fungus can happen (turning into a yeast infection which also known as candidiasis) for a variety of reasons. Hormone changes, stress, being sick or taking medication can upset the balance of the immune system and allow the yeast to multiply.
It's not uncommon for girls to get yeast infections before their periods because of the hormone changes. Also, if you are wearing clothing that traps heat and moisture, or are using scented personal products around your vagina, it can unbalance the good bacteria in your body and become an ideal environment for the infection to grow.
A yeast infection can make the vaginal area red and itchy or hurt and there may be a thick white discharge. If you have any of these symptoms, you should have a doctor check you out to properly diagnose the problem. There is treatment that will clear it up within a week.