Condoms are designed to prevent pregnancy, but they are not designed to specifically counter STDs.
Condoms Are Highly Effective in Preventing HIV Infection.
Sexual transmission of HIV occurs when infected semen, vaginal, or other body fluids contact mucosal surfaces, such as the male urethra, the vagina, or cervix. According to the Centers for Disease Control & Prevention (CDC), a number of carefully conducted studies, employing rigorous methods and measures, have demonstrated that consistent condom use is highly effective in preventing HIV transmission.
In a two-year study of sero-discordant couples (in which one partner was HIV-positive and one was HIV-negative), no uninfected partner became infected among couples using condoms correctly and consistently at every act of vaginal or anal sex versus 10 percent of those using condoms inconsistently.
In a similar two-year study, two percent of uninfected partners who used condoms consistently became HIV-infected versus 12 percent among those who used condoms inconsistently or not at all.
A recent study of declining HIV prevalence in Uganda found no evidence that abstinence or monogamy had contributed to the decline. Findings identified the increased use of condoms in casual relationships as important in Uganda's declining HIV infection rates.
Condoms Are Effective in Preventing Some STIs.(Sexually Transmitted Infection)
Gonorrhea, chlamydia, and trichomoniasis are transmitted when infected semen or vaginal fluids contact mucosal surfaces. Because condoms block the discharge of semen or protect the male urethra against exposure to vaginal secretions, condoms provide a greater level of protection against these STIs.
Condoms also provide some protection against genital ulcer STIs—such as genital herpes, syphilis, and chancroid—which are transmitted through contact with mucosal surfaces or infected skin. Because these STIs may be transmitted across surfaces not covered or protected by the condom, condoms provide a lesser degree of protection against them.
"Latex condoms, when used consistently and correctly, can reduce the risk of transmission of gonorrhea, chlamydia, and trichomoniasis."
"Genital ulcer diseases and HPV infections can occur in both male or female genital areas that are covered or protected by a latex condom, as well as in areas that are not covered. Correct and consistent use of latex condoms can reduce the risk of genital herpes, syphilis, and chancroid only when the infected area or site of potential exposure is protected. While the effect of condoms in preventing human papillomavirus infection is unknown, condom use has been associated with a lower rate of cervical cancer, an HPV-associated disease."
—CDC. Male Latex Condoms and Sexually Transmitted Diseases. Atlanta, GA: CDC, 2002.
Advocates For Youth - Condom Effectiveness