Since a lot of sex myths revolve around ways to tell if you have a cheating wife, husband, or more casual partner, I want to spend some time addressing a particularly poisonous idea. This is the sex myth that makes people believe that if their long-term partner suddenly has a new STD infection it can't have come from them, because if they were the source of the STD their partner would have been infected a long time ago.
The basic assumption lurking behind this sex myth is that if you have an STD it will be transmitted every time you have sex. That, however, isn't true. A number of factors affect the likelihood of any particular STD being spread during any particular encounter where you are having sex. These factors might include:
- The amount of virus/bacteria in your blood or in your genital secretions
- If you or your partner have any open sores or other wounds on their genitals, or in other areas where you'll have skin-to-skin contact during sex
- The presence of other STDs in either partner.
- Duration of the sexual encounter
- Whether safe sex is practiced
- Whether or not you employ lubricants, and what type of lubricants you use.
- Gender of the infected partner
- Types of sex engaged in (e.g. anal, vaginal, oral)
The absolute risk of an STD being transmitted during any given sexual encounter is highly variable. Just because your partner has only recently been diagnosed with an STD, when you have been together for a long time, does not necessarily mean that they have been cheating on you. In such a circumstance, there are several possible alternate explanations. These can be difficult to choose between, particularly if you weren't both screened for STDs prior to the beginning of the relationship. For example, if you turn out to be asymptomatically infected with the same STD, there is a possibility that you were infected the whole time and only recently transmitted the disease to your partner. Alternatively, you both could have been infected and simply not recognized the symptoms until just now.
Note: Just because STDs aren't necessarily transmitted every time you have sex doesn't mean that you can assume they won't be transmitted during any particular time when you are having sex. Your health isn't something you want to trust to luck, so it is your responsibility to do what you can to minimize your risk. Practicing safe sex, particularly when combined with appropriate testing, is a lot more reliable than relying on probability or prayer.
Baeten, J.M. et al. (2011) "Genital HIV-1 RNA Predicts Risk of Heterosexual HIV-1 Transmission" Sci Transl Med 6 April 2011 3:77ra29
Boily M.C. et al. (2009) "Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies." Lancet Infect Dis. 2009 Feb;9(2):118-29.
Fleming D.T. et al. (2006)"Herpes virus type 2 infection and genital symptoms in primary care patients." Sex Transm Dis. 2006 Jul;33(7):416-21.