It’s one thing to learn you have a sexually transmitted infection (STI). It’s quite another to learn you have an STI while you’re in a monogamous relationship.

If you have been totally faithful, you may assume that your partner acquired the infection while being unfaithful. Though it’s possible they may have been intimate with someone else, it’s also possible they never cheated at all.

This article explains two scenarios in which a partner could have an STI even though there was no infidelity in the relationship.

Preexisting Infections

One explanation for an STI that is not a result of infidelity is that you or your partner got the STI before the two of you entered into a relationship.

Many STIs do not appear right away. Usually, there is an incubation period between the moment a microbe enters the body and when symptoms begin. In fact, you may not know that you have an STI until months or even years have passed.

Average incubation periods vary among STIs.

Note that lab tests can sometimes determine when a person acquired an infection. This is particularly true of HIV and syphilis.

Treatment Failure

If you or your partner comes down with an STI, be open to the possibility that the infection is not a new one resulting from infidelity. It may actually be an old infection that wasn’t fully eliminated when it was first treated.

STIs that were thought to have been resolved after treatment can return in the future if that treatment was unsuccessful. One example of this is with syphilis—an STI that occurs in four stages: primary, secondary, latent, and tertiary.

In most cases, a syphilis infection in the primary stage can be cured with a single injection of Benzathine penicillin G. But if left untreated—or if treatment fails—the infection can advance to the secondary stage.

Once a syphilis infection has advanced to the secondary stage, it’s three times more likely that treatment will fail, compared to when syphilis is treated in the primary stage.

A person with latent syphilis will have no symptoms of infection. Logically, they may think that their treatment was successful and their infection is healed. The latent stage can last for upwards of 20 years before the symptoms of tertiary syphilis appear.

Summary

If you’re in a monogamous relationship and either you or your partner develops an STI, keep in mind that the infection may have occurred before you became a couple. An STI screen may provide answers about who infected whom and when the initial infection took place.

A Word From Verywell

In the event your partner did, in fact, acquire an STI during an encounter outside of your relationship and passed it along to you, the two of you will need to talk about what the infidelity says about your future together. Couples therapy might be helpful to make the best choice for your relationship.

This also may be a good time for you both to be screened for STIs and to practice safer sex until you’re certain that neither person is infectious.

  • Shim BS. Current concepts in bacterial sexually transmitted diseases. Korean J Urol. 2011;52(9):589-597. doi:10.4111/kju.2011.52.9.589
  • University of Michigan Health. Stages of syphilis.
  • Luo Z, Zhu L, Ding Y, et al. Factors associated with syphilis treatment failure and reinfection: a longitudinal cohort study in Shenzhen, China. BMC Infect Dis. 2017;17(1):620. doi:10.1186/s12879-017-2715-z

By Elizabeth Boskey, PhD

Elizabeth Boskey, PhD, MPH, CHES, is a social worker, adjunct lecturer, and expert writer in the field of sexually transmitted diseases.