Vaginal thrush is a common condition caused by a yeast infection in the vagina and surrounding area. Candida lives harmlessly on the skin, in the mouth, gut and vagina and is normally kept under control by harmless bacteria. But sometimes conditions change and the yeast increases rapidly, causing symptoms of discharge and irritation. Around three-quarters of women will have a bout of thrush at some point. Up to half of these will have thrush more than once.