According to a 2006 report by the National Institutes of Health, correct and consistent use of latex condoms:
• reduces the risk of HIV/AIDS transmission by approximately 85% relative to risk when unprotected. See overall HIV transmission rates.
• reduces the risk of gonorrhea for men by approximately 71% relative to risk when unprotected. ...
Other sexually transmitted infections may be affected as well, but they could not draw definite conclusions from the research they were working with. In particular, these include STDs associated with ulcerative lesions that may be present on body surfaces where the condom doesn't cover, such as genital herpes simplex (HSV), chancroid, and syphilis. If contact is made with uncovered lesions, transmission of these STIs may still occur despite appropriate condom use. Additionally, the absence of visible lesions or symptoms cannot be used to decide whether caution is needed.