Severe Vaginal Yeast Infection: Unexplained

Dr. Jeff,
Please help. My niece is 23 years old. She has had two bouts of a very severe vaginal yeast infection. This last one was the worst. Her gyno told her it was the worst one she has ever seen. All blood work came back normal. Glucose fine. She had to quit her job as a waitress, because the walking seemed to agitate it more. My sister(her mom)is scared to death. What could this be? If I could beg someone for an answer, I would.
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