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Sexually transmitted diseases (STDs) are increasingly common and service providers working in STD prevention are often asked about the chance of getting an STD from a one-time heterosexual sexual encounter.
What do studies tell us about how commonly an STD results from one-time sex? How can we interpret the results?
We summarize here what is known from studies about rates of transmission of the following STDs:
- HIV
- Genital herpes
- Chlamydia
- Gonorrhea
- Syphilis
- Ureaplasma
- Mycoplasma
- Trichomoniasis
- HPV
Males and females are equally likely to contract most sexually transmitted infections, but females are at higher risk of acquiring some of them.
Challenges in calculating the risk of HIV for heterosexual couples
It is not easy for researchers to calculate the risk of contracting HIV through sex between heterosexual couples. Studies have produced a wide range of numbers that differ on the basis of factors like the type of sexual act, the serologic status of each partner (positive or negative), and how many times the partners have had sex.
Most studies suggest that a person is more likely to contract HIV through anal sex than vaginal sex. And oral sex carries the lowest risk.
We know that there is more risk for the receptive sex partner (the person in whom the penis is inserted, anally or vaginally) than for the insertive sex partner (the person inserting the penis).
Anal sex carries the highest risk of HIV
A person can become infected with HIV from bodily fluids exchanged during anal sex. Such fluids include blood, semen, pre-seminal fluids (lubricating fluids before semen appears), and even rectal fluids. We know from the CDC and a 2018 study carried out by Patel that anal sex carries the highest risk of HIV transmission.
Women are at greater risk of contracting HIV
Researchers have found that female partners (receptive partners in vaginal sex) have a slightly higher risk of getting HIV.
The risk of HIV transmission through receptive vaginal sex ranges from 0.08% to 0.19%, according to a 2009 study by Boily and others and a 2012 secondary analysis of clinical trials performed by Hughes and others.
The risk to male partners was found to be slightly lower, 0.05% to 0.1%, by Fox and his colleagues.
Oral sex is least likely to result in HIV transmission
The risk of transmission through oral sex (whether penile–oral or vaginal–oral) is much lower than through anal or vaginal sex. This is because the oral cavity contains a thick epithelial layer, a low number of CD4 target cells, and antiviral antibodies.
A 2008 meta-analysis by Baggaley and others of 10 studies aimed at calculating the risk of HIV transmission through oral sex found that only 4 of the studies reported a non-zero estimate of risk from unprotected oral intercourse. For this reason, the CDC describes the chance of HIV transmission through oral sex as low.
Genital herpes: females’ risk is six times greater
We know that most cases of genital herpes are caused by the herpes simplex virus type 2 (HSV-2). A person who has genital herpes may have sores in multiple parts of the body. Most often, the sores are found below the waist, around the genitals or the rectum. Unfortunately, a genital herpes infection is incurable and lifelong.
We also know that genital herpes is mainly sexually transmitted via body fluids, skin, or even the genital surfaces of an infected partner.
Information from the World Health Organization shows that 14.4% of Americans had HSV-2 in 2012.
One study by Wald and his colleagues found that on average, women contract HSV-2 from 8.9 of 10,000 sexual encounters; for men, the figure is 1.5. The study analyzed data from multiple randomized controlled trials.
It is important to emphasize that women are more susceptible to the infection—up to six times more susceptible than men, according to a study by Chiron.
HSV-2 increases the risk of HIV
WHO fact sheets tell us that HSV-2 and HIV are known to affect each other. An HSV-2 infection increases the risk of getting a new HIV infection by approximately 300%.
People with both HIV and HSV-2 are more likely to spread HIV to others. HSV-2 is one of the most common infections found in HIV-positive patients—60%–90% of them have it.
Odds of getting chlamydia: again, females are at higher risk
Chlamydia trachomatis has been the most prevalent sexually transmitted disease in the United States since 1994, according to the CDC. Young females (14–39 years of age) are the most affected population in the United States.
One of the most recent and relevant studies aimed at estimating the risk of chlamydia transmission was conducted by Althaus and Low at Bern University in Switzerland in 2011.
The results of their study suggested that the chance of transmission is similar for males with infected female sexual partners and females with infected male partners. The rate of transmission is 4.5% per sexual act if one of the partners is positive. (slightly higher in female).
Females are three times more likely to get gonorrhea
A gonorrhea infection is caused by Neisseria gonorrhea bacteria. It can be sexually transmitted by vaginal, oral, or anal means. After just one episode of sex with an infected partner, a female has a 60% to 90% chance of being infected by a male, while a male’s risk of being infected by a female is only 20%.
This huge difference may be related to the larger exposed genital surface area in females as well as females’ greater exposure to trauma and tissue damage.
Syphilis transmission through a single heterosexual contact
Syphilis transmission is usually sexually transmitted. The risk of contracting syphilis from a sexual partner depends on many factors, including the frequency of sex, type of sexual contact (i.e. penile–vaginal, penile–anal, or penile–oral), the stage of syphilis in the infected patient, the susceptibility of the partner, and whether a condom is used.
Stoltey and Cohen reported in a 2015 review article that the probability of transmission of a syphilis infection to a heterosexual partner through a single sexual act is 51%–64%.
Ureaplasma transmission: more likely in women with multiple sexual partners
Ureaplasma infection can be caused by either Ureaplasma parvum or Ureaplasma urealyticum . We know that both naturally live on and in the human body. Under some circumstances, atypical bacteria (new mutated generations) can occur and colonize the body, causing an infection.
People can acquire the atypical bacteria through vaginal and oral sexual acts, which is why they are called opportunistic pathogens.
Research by Kokkayil and Dhawan in 2015 showed that the rate of vaginal colonization (aggregation) of Ureaplasma is from 8.5% to 77.5%. The colonization rate was found to be associated with having multiple sexual partners, and the transmission rate per sexual act is still unknown.
Mycoplasma transmission
Mycoplasma genitalium is a well-known sexually transmitted pathogen. The prevalence of Mycoplasma genitalium infection among men and women in the United States and United Kingdom is estimated to be 1%–3%.
Studies have shown that this pathogen is responsible for most cases of nongonococcal urethritis.
Contracting this infection increases the likelihood of many complications, especially in women. Some potential complications include cervicitis, pelvic inflammatory disease, preterm delivery, and spontaneous abortion.
Little data is available regarding the probability of mycoplasma transmission through sexual acts with an infected partner. In a 2017 review article, Slifirski and his colleagues reported, on the basis of data from multiple small studies, that the likelihood of contracting mycoplasma infection through a single sexual act is 20.6%–66.7%.
Trichomoniasis odds
Studies by the CDC show that Trichomoniasis is the most common treatable sexually transmitted disease. It is caused by a protozoan parasite, Trichomonas vaginalis.
From studies in STD clinics, we know that the prevalence of Trichomoniasis is 13%–34% among women and 3%–17% among men.
Trichomonas is a highly communicable disease. Between 14% and 60% of male partners of infected females are infected, and 67%–100% of female partners of infected males are infected.
HPV transmission rates
There are many serotypes of human papillomavirus (HPV); the most important are HPV 16 and 18. These are recognized as the main cause of cervical cancer.
There are many studies of the prevalence of HPV among heterosexual couples. One of the most respected and recognized is the HITCH study by Ann N. Burchell in 2011. Burchell followed newly formed heterosexual couples from 2005 to 2010 to determine the transmission rate in relation to time. Researchers found that the male-to-female transmission rate was 3.5% per 100 person-months and the female-to-male transmission rate was 4% per 100 person-months.
Conclusion
Millions of new cases of sexually transmitted infections are diagnosed in the United States and Europe every year. While chlamydia is the most prevalent, it is important to know that HIV is the most serious. Males and females are at similar risk of contracting most sexually transmitted infections, but females are more likely to acquire some STDs, such as gonorrhea and trichomoniasis.
