STIs and Infidelity

If you become infected, it may not mean your partner cheated

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. Provided you've been totally faithful yourself, you may logically assume the source of your infection is your partner—and that they must have been intimate with someone else, picked up an STI, and then shared it with you.

It may be true that's exactly what has happened. But it also could be your partner hasn't been within a mile of someone else sexually. There are some scenarios in which one person in a monogamous relationship develops an STI without having been cheated on.

If you find yourself in this position—diagnosed with an STI that your partner swears could not possibly be the result of infidelity on their part—be open to this possibility. By understanding why this could happen, you may well discover that what your loved one is telling you is true.

Couple looking serious on the bed
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Preexisting Infections

An STI your partner swears couldn't possibly be the result of infidelity—because they have been absolutely faithful—could actually be one that either you or they contracted before the two of you entered into a monogamous relationship.

Many STIs are not instantaneously apparent. Usually, there is an incubation period between the moment a microbe enters the body and the development of obvious symptoms. In fact, an infection can take hold months or even years before a person knows about it.

Average incubation periods vary dramatically among STIs.

Incubation Periods for Common Sexually Transmitted Infections
STI Incubation Period
Genital herpes 2 days to 14 days
Chlamydia 14 days to 21 days
Trichomoniasis 5 days to 28 days
Gonorrhea 5 days to 30 days
Hepatitis B 28 days to 42 days
Syphilis 10 days to 90 days
HIV 2 weeks to several years

Note that lab tests can sometimes pinpoint when an initial infection took place. This is particularly true of HIV and syphilis.

Treatment Failure

An STI that occurs in a monogamous partner may actually be the recurrence of a previous infection that was treated—but unsuccessfully. Some STIs that are thought to have been resolved after treatment may defy the odds.

Advanced (secondary) syphilis is one example. Early syphilis is almost always cured with a single injection of Benzathine penicillin G. But there is a three-fold risk of treatment failure in people with secondary syphilis compared to those with primary syphilis, according to a 2017 study in BMJ Infectious Diseases.

A post-treatment follow-up test can confirm the syphilis infection is totally cleared. But if such a test isn't performed, a person may go for years believing they're infection-free until symptoms arise.

If the STI you come down with happens to be one that you or your partner had years earlier and received treatment for, be open to the possibility that the infection is not a new one resulting from infidelity, but rather an old one that wasn't fully removed when first treated.

Resolving Dilemmas

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

In the event your partner did, in fact, become infected with an STI during an encounter outside of your relationship and passed it along to you, the two of you will need to have a conversation (or several) 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 or re-screened for STIs and to practice safer sex until you're certain you both are infection-free (or, in the case of STIs that are incurable, undergoing treatment and are not infectious).

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  1. Shim BS. Current concepts in bacterial sexually transmitted diseases. Korean J Urol. 2011;52(9):589-97. doi:10.4111/kju.2011.52.9.589

  2. 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

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