Vaginal thrush (thrush) is a yeast infection caused by a type of fungus of the candida species, usually Candida albicans. It can affect all women, but is more common among women who are pregnant, those who have weakened immune systems, and women aged 30 to 50. Thrush is generally recurring - it comes back. The fungus, candida albicans, exists naturally in the vagina. As long as it does not multiply too much a woman will not notice it is there. However, if can sometimes multiply to such an extent that it causes swelling of the vagina and vulva.