Do condoms prevent STDs?

Male latex condoms can reduce your risk of getting an STD if used correctly (see the box below). Be sure to use them every time you have sex, during all types of sex, including vaginal, anal and oral sex. Female condoms aren't as effective as male condoms, but should be used when a man won't use a male condom.
Remember, though, that condoms aren't 100% safe and can't protect you from coming in contact with some sores, such as those that can occur with herpes, or warts, which can occur with HPV infection.

Should I use a spermicide to help prevent STDs?

No. It was once thought that spermicides with nonoxynol-9 could help prevent STDs much like they help prevent pregnancy -- by damaging the organisms that cause the diseases. New research has shown that nonoxynol-9 can also irritate a woman's vagina and cervix, actually increasing the risk of STD infection.
Be sure to also check the ingredients of any other sex-related products you own, such as lubricants and condoms. Some brands of these products may have nonoxynol-9 added to them. If you are unsure if your spermicide or any other product contains nonoxynol-9, ask your doctor before using it.