Laboratory tests show that no STI, including HIV, can penetrate an intact latex condom (111, 112, 162, 284, 446, 494). (Infectious organisms can sometimes pass through condoms made from lamb's intestine, often called natural skin condoms, so they should be used only for contraception, not infection prevention (86, 363).)

Protection against HIV/AIDS. Condoms provide highly effective protection against HIV infection when used correctly with every act of intercourse. All 10 cohort studies conducted through 1995 that evaluated condom use among heterosexual couples showed that consistent condom use protected against HIV (166).

The most convincing evidence of condom effectiveness comes from studies of HIV-discordant couples—couples in which one partner is infected with HIV and the other is not (166, 557). Studies of such couples have found low risks of HIV infection among consistent condom users (22, 166, 167, 177, 311, 378, 416, 557). In three recent studies infection rates were less than 1% per year among consistent condom users (134, 141, 473). A multicountry European study of 256 HIV-discordant couples followed for an average of 20 months found that not one infection occurred among such couples using condoms during every sex act (134).

Widespread and consistent use of condoms could reduce the number of people infected with HIV enough to slow the spread of HIV/AIDS (410). Condoms may help prevent AIDS over the long term not only by blocking transmission of HIV but also by protecting against other STIs. People with STIs, particularly those that cause genital ulcers—chancroid, genital human papillomavirus (HPV), herpes simplex, and syphilis—are two to seven times more likely to become infected with HIV than people who do not have STIs (142, 306, 307, 401, 416).

Some protection against other STIs. Since the 1970s studies of typical use have demonstrated that condoms provide some protection against gonorrhea, syphilis, and chlamydia (39, 46, 85, 86, 131, 253, 346, 400, 492). For example, consistent condom use probably reduces the risk of chlamydia and gonorrhea infection by about 60% to 80% (471). One study found that condoms reduce the risk of trichomoniasis by about 30% (457). Condoms offer less protection against herpes, the genital wart virus HPV implicated in cervical cancer, and other STIs that can be transmitted through skin-to-skin contact between parts of the body not covered by condoms (86, 93, 146, 300).

Condoms treated with the most commonly used spermicide, nonoxynol-9, apparently offer no greater protection against STIs than do other condoms (162). Although spermicides are effective against common STIs, the amount on condoms is only about one-third of that in vaginal spermicides and may not be enough to prevent STI transmission (26, 533). A recent study of commercial sex workers in the Dominican Republic found no difference in STI rates between women using condoms lubricated with nonoxynol-9 and women using silicone-lubricated condoms (454).

Protection against pelvic inflammatory disease and ectopic pregnancy. By preventing STIs, condoms also help to prevent pelvic inflammatory disease and its consequences, infertility and ectopic pregnancy (pregnancy outside the uterus) (285, 319). Ectopic pregnancy is a life-threatening condition requiring surgery, and it causes infertility in perhaps as many as two-thirds of women who survive (225).