Unprotected sexual intercourse - how high is the risk of an STI?

Unprotected sexual intercourse carries a high risk of sexually transmitted infections (STIs). STIs such as chlamydia, gonorrhoea, HIV, or syphilis can easily be transmitted through unprotected sex. Individuals with frequently changing sexual partners, those who practice anal sex, and those who do not undergo regular screening are particularly at risk. This article explains how STIs are transmitted, which risk factors are involved, and how you can effectively protect yourself.

Table of Contents

• What are STIs?
• Why is protection during sex important?
• How high is the risk of STI transmission during unprotected sex?
• Which factors influence the risk of STI transmission?
• How can I prevent an STI infection?
• Summary

What are STIs?

Sexually transmitted infections (STIs) are infections transmitted through unprotected sexual contact and are classified into three categories depending on the type of pathogen. Bacterial infections include chlamydia, gonorrhoea, and syphilis. Viral infections include HIV, genital herpes, and HPV, while parasitic infections such as trichomoniasis may also occur. If left untreated or undetected, STIs can lead to serious health complications (MedlinePlus, 2024).

Why is protection during sex important?

Protection such as condoms and dental dams is important during sexual activity for the following reasons:

• Protection reduces the risk of STI transmission by limiting direct contact with infectious bodily fluids and mucous membranes. When used correctly and consistently, condoms can reduce the risk of HIV transmission by up to 99%.
• Protection not only safeguards your own health but also prevents unknowingly passing infections to others, as many STIs can be asymptomatic.
• Long-term complications from untreated and undiagnosed infections can be prevented through consistent protection.
• The overall spread of STIs in the population is reduced. Preventive behaviour helps break the chain of infection.

Source: NHS, 2022

How high is the risk of STI transmission during unprotected sex?

The risk of acquiring a sexually transmitted infection (STI) during unprotected sex varies depending on the type of infection, mode of transmission, and individual factors such as mucosal health or existing infections.

• Human Immunodeficiency Virus (HIV): The risk for vaginal intercourse ranges between 0.04–0.08%, while for anal intercourse it can be as high as 1.4%. Contact with infected fluids such as blood, semen, or vaginal secretions is critical. Mucosal injuries or a high viral load significantly increase the risk.
• Chlamydia and Gonorrhoea: The risk of infection per sexual contact is around 20–40%. These bacteria primarily spread through vaginal, anal, and oral sex by affecting the mucous membranes and through direct contact with infected secretions.
• Genital Herpes: Herpes simplex viruses (HSV) spread through skin-to-skin contact and infected mucous membranes, even when no visible symptoms like blisters are present. The risk per encounter ranges from 10–30%, especially high during outbreaks with open blisters.
• HPV: Human papillomaviruses are transmitted through direct skin or mucous membrane contact, often during vaginal, anal, or oral sex. The infection risk is difficult to assess, as HPV is often asymptomatic. Around 80% of sexually active individuals are infected with HPV at some point in their lives (Lewis et al., 2019).
• Syphilis: Syphilis is transmitted through direct contact with open syphilitic sores on the skin or mucous membranes. The transmission risk per contact is 30–50%. The infection is most contagious in the early stages when visible lesions are present.
• Trichomoniasis: The parasite Trichomonas vaginalis, which causes trichomoniasis, is mainly transmitted through vaginal sex. The infection risk can be as high as 70%, as it is highly contagious. Men often carry the infection without symptoms.

Sources: Mayo Clinic, 2023; NHS, 2022

More information can be found in our blogs “Sexually Transmitted Infections (STIs): Myths and Facts – What’s Your Real Risk?” and “Sex Without a Condom – Risks and Common Questions”.

Which factors influence the risk of STI transmission?

The risk of contracting an STI during unprotected sex is influenced by the following factors:

• Type of sexual activity: The mode of transmission greatly affects the risk. Unprotected anal sex carries the highest risk due to the fragility of the anal mucosa, followed by vaginal and oral sex.
• Partner status: If your sexual partner is infected with an STI, your risk of infection increases significantly. It is particularly risky if the partner’s STI status is unknown and there are frequently changing partners. Many STIs are asymptomatic but still transmissible and may cause long-term harm.
• Lesions/micro-injuries: Open wounds, irritated skin, or small unnoticed injuries facilitate the entry of pathogens such as bacteria or viruses.
• Type of STI and infectiousness: Some infections spread more easily than others due to their transmission routes. HPV, for instance, is more easily transmitted through skin contact than HIV, which requires exposure to fluids like blood or semen. Additional factors include how long the pathogen can survive outside the body, the viral load, and whether the infection shows symptoms.
• Recipient’s immune system: Individuals with weakened immune systems—due to chronic illnesses, HIV, or immunosuppressive medications—are more susceptible to infection.
• Social and geographic factors: In regions with high STI prevalence or among high-risk groups (e.g. sex workers, men who have sex with men), the likelihood of infection is significantly increased.

Source: Mayo Clinic, 2023

How can I prevent an STI infection?

To prevent sexually transmitted infections or avoid unknowingly passing them on, always use condoms during vaginal, anal, and oral sex. Regular testing is also recommended, especially before starting a new relationship, when having multiple partners, or after unprotected sex. Vaccination—particularly against high-risk HPV types—is advisable. Open and honest communication with your partner about sexual health is essential to protect each other and to decide on testing or treatment together. Avoid sexual contact if you are experiencing symptoms or suspect an STI (NHS, 2023).

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Summary

Unprotected sex carries a high risk of contracting sexually transmitted infections (STIs) such as chlamydia, gonorrhoea, HIV, or syphilis. Individuals with frequently changing sexual partners or those who practice anal sex without regular screening are particularly at risk. Protective measures such as condoms and regular testing are essential to reduce the risk of infection and prevent unknowingly spreading STIs. Vaccination against high-risk HPV types and open communication with partners further contribute to effective prevention.