Bacterial Vaginosis-- (BV) is not strictly an STD because it is not transmitted by via sexual intercourse. However, it can be aggravated  by sex and is more frequently found in sexually active women than those who are not. It is caused by an imbalance in the normal healthy bacteria found in the vagina and although it is relatively harmless and may pass unnoticed, it can sometimes produce an unpleasant fishy smelling discharge.

While there is no clear explanation as to why BV occurs, there have been suggestions that the alkaline nature of semen could be one cause, as it may upset the acidic nature of the vaginal bacteria. Another cause can be the use of an intrauterine contraceptive device (coil). A woman cannot pass BV to a man, but it is important she receives treatment as BV can occasionally travel up into the uterus and fallopian tubes and cause a more serious infection. Treatment for BV consists of applying a cream to the vagina or taking antibiotics.