I get a lot of e-mails from people wanting to know the odds of getting an STD if they have sex with a specific type of person or in a particular way. However, that's not as easy a question to answer as they seem to think. The odds of getting an STD depend on a number of factors, including:
- How you have sex (i.e., manual, anal, vaginal, oral)
- Whether you use condoms or other barriers to practice safe sex
- If you use sexual lubricants, and what kinds of lubricants you use
- Whether your partner has an STD
- If they do, what STD they have
- The severity of their infection, as measured by viral load and other factors
- Your overall health and the health of your immune system
- Whether you have breaks in your skin or other STDs that might make you more susceptible to infection
Then, assuming those things were all known, scientists would simply need to know the odds of transmitting the STD in question, during the particular type of sex you're having, with all of the other variables also taken into account, in order to give you an idea of the actual odds of getting an STD in any particular sexual encounter.
But they don't.
Although there is some research into the odds of transmitting HIV during various types of intercourse, and it's possible to tell that suppresive therapy reduces herpes transmission, it's really difficult to design a study that will tell scientists exactly how likely it is that an STD will be transmitted any particular time a person has sex.
That's why I, personally, won't answer this type of question at all. I can't accurately tell you what the odds are of getting an STD the next time you have sex, and I won't even try. All I can do is help you be aware of the risk by getting screened and talking to your partner before you have sex... and help you reduce your risk by encouraging you to reliably practice safe sex.
Your health, and your partner's health, shouldn't be a numbers game -- particularly when the numbers are as unreliable as they are. It's like playing Russian Roulette with your reproductive tract, although with a gun you can at least count the number of bullets and calculate your actual odds. There's no good way to do that with STDs.
Remember: Just because STDs aren't transmitted every time a person has sex doesn't mean they won't be transmitted the next time you have sex.