The question of how a person can acquire a sexually transmitted infection (STI) when their partner seemingly does not have one is common. STIs, often referred to as STDs, are caused by bacteria, viruses, or parasites passed from one person to another, most often through sexual contact. The premise that a partner is clear of infection is frequently based on a lack of visible symptoms or a previous negative test result. However, the biological nature of many STIs and the limitations of current diagnostic methods mean that an infection can be present and transmissible without either partner knowing. Understanding these factors is necessary for accurate risk assessment and informed sexual health decisions.
Infection Without Visible Symptoms
The primary reason transmission occurs when a partner appears healthy is the high rate of asymptomatic carriage for many common STIs. A person may be infected and contagious without ever experiencing discomfort or developing visible lesions. This is particularly true for bacterial infections like Chlamydia and Gonorrhea, which can remain asymptomatic for months or even years, especially in women.
The human papillomavirus (HPV), the most prevalent STI globally, is also frequently asymptomatic. Viral infections like Herpes Simplex Virus (HSV), which causes genital herpes, can also be present without obvious outbreaks. Up to 90% of people with HSV-2 may be unaware they have the virus, making transmission possible even when there are no sores.
During these symptom-free periods, the virus or bacteria can still be released from the body through a process called viral or bacterial shedding. Asymptomatic shedding is believed to be the most common way HSV-2 is transmitted. The infectious agent is present on the skin or mucous membranes, often for brief periods, and can be passed to a partner through contact. This means a person can genuinely believe they are infection-free while actively transmitting the STI.
Limitations of Testing and Diagnosis
Another significant reason for confusion is that a partner may have received a false negative test result, leading to the mistaken belief that they are clear. This diagnostic inaccuracy is often related to the timing of the test relative to potential exposure. Every STI has an incubation period—the time between exposure and when the body produces enough of the pathogen or corresponding antibodies to be detected by a standard test.
Testing too early, during this “window period,” can easily lead to a false negative result, meaning the infection is present but undetected. For example, Chlamydia and Gonorrhea tests may be unreliable if performed within the first week of exposure. Some tests for HIV may require waiting up to three months for a definitive result. The window period varies significantly depending on the specific STI and the type of test used, making it necessary to consider the exposure date when interpreting results.
The accuracy of the testing method itself also plays a role in generating false negatives. Different tests have varying levels of sensitivity, which is the ability to correctly identify an infected person. Nucleic Acid Amplification Tests (NAATs) are highly sensitive for bacterial infections like Chlamydia and Gonorrhea, as they look for the genetic material of the pathogen. However, antibody tests for viruses like HIV or Hepatitis require the immune system to have built up a detectable response, which takes time. The possibility of human error during sample collection or laboratory processing can further compromise the reliability of an otherwise accurate test.
Non-Sexual Transmission Pathways
It is also possible for an STI to be acquired through a route entirely unrelated to the current partner or any sexual activity. While sexual contact is the most frequent transmission pathway, certain infections can be spread non-sexually. Bloodborne pathogens, such as HIV and Hepatitis B and C, can be transmitted through the sharing of contaminated needles, which is common in intravenous drug use.
Another non-sexual route is vertical transmission, where an infection is passed from a pregnant mother to her baby during pregnancy or childbirth. This is a known pathway for infections like Syphilis, Chlamydia, and HIV.
Some STIs can also be transmitted through close skin-to-skin contact, even without penetrative sex. Herpes, for instance, can be spread through kissing if one person has an active cold sore, which is caused by the Herpes Simplex Virus. Although less common, certain infections can be passed through contact with infected blood or fluids outside of sexual activity.