Infection occurs when new yeast is introduced into the vaginal area, or when there is an increasing number of yeast already present in the vagina relative to the amount of normal bacteria. For example, the normal, protective bacteria were eradicated by antibiotics (taken to treat urinary tract, respiratory, or other type of infection) or with immunosuppressive drugs, the yeast can multiply, invade tissues and cause irritation of the lining of the vagina (vaginitis).
Vaginal yeast infections can also occur as a result of injury to the inner vagina, such as after chemotherapy. Also, women with suppressed immune systems (for example, taking cortisone-related medications such as prednisone) develop vaginal yeast infections more often than women with normal immunity. Other conditions that can affect women to develop vaginal yeast infections include diabetes mellitus, pregnancy, and taking oral contraceptives. The use of douches or perfumed sprays clean the vagina may also increase a woman's risk of vaginal yeast infections.
Vaginal yeast infection is not considered a sexually transmitted infection (STD), since Candida may be present in the vagina is normal, and the condition does not occur in celibate women. However, it is possible for humans to develop symptoms of skin irritation of the penis from a yeast infection after intercourse with an infected partner.
source : from science book