What std causes a fishy odor?

A vaginal discharge with a fishy odor that gets stronger after sex or after using soap is usually bacterial vaginosis (BV). BV is not a sexually transmitted disease; if you have it, your partner does not need to be treated. Sometimes, BV can be associated with trichomoniasis, though, so if the odor doesn't go away in a few days, if you have any pain, or if you're pregnant, see your health care provider for treatment of BV and to check that there's not some other problem.