has to say on the subject of Condom Effectiveness. (See Related Link for the full fact sheet for Public Health Personal.)
"The surest way to avoid transmission of sexually transmitted diseases is to abstain from sexual intercourse, or to be in a long-term mutually monogamous relationship with a partner who has been tested and you know is uninfected.
"For persons whose sexual behaviors place them at risk for STDs, correct and consistent use of the male latex condom can reduce the risk of STD transmission. However, no protective method is 100 percent effective, and condom use cannot guarantee absolute protection against any STD. Furthermore, condoms lubricated with spermicides are no more effective than other lubricated condoms in protecting against the transmission of HIV and other STDs. In order to achieve the protective effect of condoms, they must be used correctly and consistently. Incorrect use can lead to condom slippage or breakage, thus diminishing their protective effect. Inconsistent use, e.g., failure to use condoms with every act of intercourse, can lead to STD transmission because transmission can occur with a single act of intercourse."
A report on The Straight Dope, "Do condoms protect against STDs other than HIV? STD-wise, how risky is oral sex?" ( http://www.straightdope.com/columns/read/2264/do-condoms-protect-against-stds-other-than-hiv ) says pretty much the same things as the CDC Fact Sheet and is easier to read.
The SD report begins, 'Let's start off with a basic premise, as laid out last year in the New England Journal of Medicine: "[T]he protection that condoms offer against a specific sexually transmitted infection cannot be precisely quantified."'
As one might expect, the best studies have been done on HIV since it is by far the most dangerous STD.
There is a fairly accepted number for risk reduction with HIV using the male latex condom. The CDC article quoted above does not give the number, but the Wikipedia article I cited in an earlier post says the risk reduction is 85%. That is cited and agrees with what I've read in other places.
The problem, which is rarely stated outside the professional journals, is that the risk reduction is 85% EACH TIME YOU USE A CONDOM. Thus if you have sex twice using a (different, of course) condom each time, the risk reduction is 0.85 * 0.85 or approximately 0.72. If you have sex a third time (also using a condom) the total risk reduction is 0.72 * 0.85 or approximately 0.61. You can see where this is going. If you depend on condoms for protection AND your partner is infected, you will ultimately get infected. On average, it will take longer. With a risk reduction of 85%, on average it will take seven times as long to become infected, but IT WILL HAPPEN.
It was stated in an earlier post that a second grader could do the risk analysis. I'm not convinced the average second grader would correctly do this analysis.
A condom is great for a single encounter with a partner of unknown STD status, but I wouldn't depend on them to keep me healthy over an extended period of time.
The only real safety comes from having partners who are not infected with anything. In real life it is impossible to be certain that your partner is disease free. However, you can try to avoid the HIV high-risk groups (men who have sex with men, injecting drug users, and anyone who has sex with anyone in the first two groups or with a partner known to be infected). This should be helpful, at least with HIV.
Personally, I feel that a recent STD test gives more protection than a condom, but that is a personal choice based on my own risk tolerance and assessment of the risks.