What are the odds of contracting 9 different STDs from a single heterosexual encounter?

HIV

Calculating the risk of contracting HIV through sexual activity between heterosexual couples, where one partner is HIV positive, presents several challenges for researchers. Studies have shown varying results depending on factors such as the type of sexual act, the viral load of the infected individual, and the duration of intercourse.

Most studies indicate that anal sex carries a higher likelihood of transmitting HIV compared to vaginal sex, while oral sex presents the lowest risk.

It is understood that the receptive partner (the individual receiving the penis, whether rectally or vaginally) faces a greater risk than the insertive partner (the individual inserting the penis).

Anal sex has the highest risk of HIV transmission. Infection can occur through bodily fluids exchanged during anal sex, including blood, semen, pre-seminal fluids (the lubricating fluids released before sperm appears), and even rectal fluids. According to the CDC and a 2018 study by Patel, anal sex poses the greatest risk for HIV transmission.

Studies show that female partners (the receptive partners during vaginal sex) have a slightly higher risk of acquiring HIV.

The risk of HIV transmission through receptive vaginal sex ranges from 0.08% to 0.19%, based on a 2009 study by Boily and colleagues and a 2012 secondary analysis of clinical trials conducted by Hughes and others. Male partners' risk is slightly lower, ranging from 0.05% to 0.1%. [1] Heterosexual risk of HIV-1 infection per sexual act: systematic review and meta-analysis of observational studies https://pubmed.ncbi.nlm.nih.gov/19179227/

The risk of transmission through oral sex — specifically receiving oral sex—is significantly lower than through anal or vaginal sex. The oral cavity has a thick epithelial layer, fewer CD4 target cells, and contains antiviral antibodies.

A 2008 meta-analysis by Baggaley and others reviewed 10 studies calculating the risk of HIV transmission through oral sex, finding that only 4 of those studies reported any measurable risk from unprotected oral intercourse. Consequently, the CDC describes the likelihood of HIV transmission through oral sex as low.

Chlamydia

Chlamydia can be transmitted through vaginal, oral, or anal sex.

In 2011, Althaus and Low conducted a significant study at the University of Bern in Switzerland to estimate the risk of chlamydia transmission. [2] Transmission of Chlamydia trachomatis through sexual partnerships: a comparison between three individual-based models and empirical data https://pubmed.ncbi.nlm.nih.gov/21653569/ Their findings revealed that the likelihood of transmission is similar for males with infected female partners and females with infected male partners. The transmission rate is 4.5% for each sexual act when one partner is infected.

Gonorrhea

Gonorrhea can be transmitted through vaginal, oral, or anal sex. After just one encounter with an infected partner, a female has a 60% to 90% chance of contracting gonorrhea from an infected male, while a male’s risk of infection from an infected female is only 20%. This significant difference may be due to the larger exposed genital surface area in females and their increased susceptibility to trauma and tissue damage.

Herpes

Herpes can be transmitted even when there are no visible sores. An infected partner may have a history of herpes and still not show symptoms at the time of contact.

It is crucial to note that women are significantly more susceptible to the infection—up to six times more than men, according to a study by Chiron. [3] A prospective study of new infections with herpes simplex virus type 1 and type 2. Chiron HSV Vaccine Study Group https://pubmed.ncbi.nlm.nih.gov/10547406/

Another study by Wald and colleagues found that women contract HSV-2 in 8.9 out of 10,000 sexual encounters, while men have a rate of 1.5 per 10,000 encounters. This analysis drew from multiple randomized controlled trials.

According to the World Health Organization (WHO), 14.4% of Americans had HSV-2 in 2012. WHO fact sheets also indicate that HSV-2 and HIV affect each other.

An HSV-2 infection raises the risk of acquiring a new HIV infection by about 300%. Those with both HIV and HSV-2 are more likely to transmit HIV to others. HSV-2 is highly prevalent among HIV-positive patients, with 60% to 90% carrying the virus.

Syphilis

The risk of contracting syphilis from a sexual partner is influenced by several factors, including the type of sexual contact (such as penile-vaginal, penile-anal, or penile–oral), the stage of syphilis in the infected person (most infections occur during the first stage when an ulcer is present), and the susceptibility of the partner.

According to a 2015 review article by Stoltey and Cohen, the likelihood of transmitting syphilis to a heterosexual partner after a single sexual encounter with an infected person ranges from 51% to 64%. [5] Syphilis transmission: a review of the current evidence https://pubmed.ncbi.nlm.nih.gov/25702043/

Trichomoniasis

Research from STD clinics shows that the prevalence of Trichomoniasis ranges from 13% to 34% in women and from 3% to 17% in men.

The likelihood of transmitting Trichomoniasis during a single sexual encounter with an infected person is high, ranging from 60% to 90%. Furthermore, the risk of contracting the infection is more significant for females.

Mycoplasma

The prevalence of Mycoplasma genitalium infection among men and women in the United States and the United Kingdom is estimated to be between 1% and 3%.

Studies indicate that this pathogen is responsible for the majority of cases of nongonococcal urethritis.

Limited data is available regarding the risk of mycoplasma transmission during sexual activities with an infected partner. In a 2017 review article, Slifirski and colleagues pointed out that, based on data from several small studies, the likelihood of contracting a mycoplasma infection from a single sexual encounter ranges from 20.6% to 66.7%. [6] Mycoplasma genitalium Infection in Adults Reporting Sexual Contact with Infected Partners, Australia, 2008-2016 https://pubmed.ncbi.nlm.nih.gov/29047422/

Ureaplasma

Ureaplasma is an opportunistic pathogen that can inhabit the vagina without necessarily causing any symptoms. The likelihood of colonization increases with the number of sexual partners, although the exact transmission rate during a single sexual encounter is still unclear. Research conducted by Kokkayil and Dhawan in 2015 found that the vaginal colonization rates of Ureaplasma vary significantly, ranging from 8.5% to 77.5%. [7] Ureaplasma: current perspectives https://pubmed.ncbi.nlm.nih.gov/25865969/

Currently, there is limited information available about the risk of transmitting Ureaplasma during one sexual encounter with an infected partner.

HPV

Numerous studies have investigated the prevalence of HPV among heterosexual couples. One of the most respected is the HITCH study, conducted by Ann N. Burchell in 2011. Burchell tracked newly formed heterosexual couples from 2005 to 2010 to assess the transmission rate over time.

The average likelihood of acquiring HPV from a single sexual encounter is approximately 40%. However, the transmission rate can vary widely, ranging from 5% to 100%.