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Causes, Risk Factors and Prevention of STDs

By Jill Grimes, M.D., FAAFP ; Updated August 14, 2017

Sexually transmitted infections (STIs, also known as sexually transmitted diseases, or STDs) are largely spread exactly how the name suggests — through sexual intimacy. However, although people often believe STIs are only spread through intercourse, the reality is that STIs are easily spread through oral sex, anal sex and vaginal intercourse. Some STIs, such as pubic lice (“crabs”), can be spread by contaminated clothing or linens, so no sex of any type is required for transmission. Other STIs, such as herpes, syphilis and HPV, can be spread by direct skin-to-skin contact with an infected partner, even without intercourse. Lastly, pregnant mothers infected with an STI can directly pass on this infection to their babies during pregnancy or childbirth.

The major risk factor for catching a sexually transmitted infection is being sexually intimate with another person, and the greater number of partners a person has, the greater their numerical risk of developing an STI. On the other hand, remaining celibate or in a long-term mutually monogamous relationship with an uninfected partner minimizes risk. However, there are other risk factors that are less obvious.

Condom Use

“Unprotected sex” has traditionally referred to intercourse without a condom, and “safe sex” suggests intercourse with a condom. Since several common STIs (such as herpes and HPV) can be spread from skin beyond the area covered by a condom, “safer sex” is a more accurate term. Condoms dramatically reduce the risk of transmitting STIs that are spread through semen and vaginal fluid, including HIV, gonorrhea, chlamydia and trichomoniasis. Condoms also significantly decrease but cannot eliminate transmission of herpes, HPV and syphilis. However, condoms must be used correctly and consistently to decrease risk of STIs. Specifically, condoms should be used for all types of genital intimacy, including oral, anal and vaginal penetration, and they must be placed before any pre-ejaculate is released.

Alcohol or Drug Use

Not surprisingly, using any mind-altering substance, such as drugs or alcohol, increases risk for STIs (through impaired judgment regarding personal sexual-intimacy limits and choices regarding condom use).

Age and Physical Predisposition

Of the more than 20 million new STIs estimated each year in the United States, more than half occur in young people ages 15 to 24.

Adolescent females are at higher risk of acquiring some STIs because the area with the most vulnerable cells on their cervix (entrance to the womb) is larger and more exposed than in older women.

Prior STI

Having a previous STI increases the risk of getting another STI:

  1. People are often re-infected with the same disease because their partner was not treated properly, or they were intimate again before both partners had completed a full course of medication.

  2. When one STI is present, it is much easier to catch a second STI, because there is irritation and inflammation that disrupts the lining of the vagina or anus, breaking down the tissue around these areas (so the usual barrier is not intact), allowing the second infection to enter the body more easily. For example, studies show that having an untreated trichomonas vaginal infection can triple a woman’s risk of contracting HIV if she were to have sex with an HIV-infected partner.

  3. STIs frequently occur together, meaning you are infected with two or more STIs at the same time because they are transmitted the same way. For example, when someone tests positive for gonorrhea, the CDC recommends to also treat the patient for chlamydia.


If a pregnant woman is infected with an STI, this infection may be passed on to her baby either during pregnancy or during a vaginal delivery. Routine prenatal screening for STIs helps clinicians detect and treat these infections as early as possible to minimize risk to the baby.

Men With Erectile Dysfunction

Studies show that men taking erectile dysfunction medications have higher rates of STIs. In a frustrating loop, men with ED often avoid using condoms, thus increasing their risk of acquiring STIs, and STIs can cause or worsen ED through both physical and psychological means.

Next Steps

STIs are very often silent, so the only way to know if you have one is to get tested. Talk with your doctor about STI testing if you have been sexually intimate with a new partner since your last exam.

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