STDs and Infidelity

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

Couple looking serious on the bed

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It's one thing to learn you have a sexually transmitted disease (STD). It's quite another to learn you have an STD 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 STD, 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 in a sexual way. There are some scenarios in which one person in a monogamous relationship develops an STD without having been cheated on.

If you find yourself in this position—diagnosed with an STD 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.

Pre-existing Infections

An STD 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.

This is because many sexually transmitted infections 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 sexually transmitted diseases.

Incubation Periods for Common Sexually Transmitted Diseases
STD Incubation Period
Genital Herpes Two days to 14 days
Chlamydia 14 days to 21 days
Trichomoniasis Five days to 28 days
Gonorrhea Two days to 30 days
Hepatitis B 28 days to 42 days
Syphilis 10 days to 90 days
HIV Two 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 STD that occurs in a monogamous partner may actually be the reoccurrence of a previous infection that was treated—but unsuccessfully. Some STDs 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 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 STD 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 obliterated when first treated.

Resolving Dilemmas

If you're in a monogamous relationship and either you or your partner develops an STD, keep in mind the possibility that it could be the result of an infection that occurred before you became a couple. An STD 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 STD 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.

This also may be a good time for you both to be screened for STDs (even if you already have done this) and to practice safer sex until you're certain you both are infection-free (or, in the case of STDs that are incurable, are undergoing treatment and are not infectious).

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