Causes and Risk Factors of STIs

Sexually transmitted infections (STIs) are caused by intimate contact with a partner living with an STI. The more sexual partners you have, the more likely you are to be exposed to an STI. But there are other risk factors as well. The type of sex and sexual partners you have, whether you practice safer sex consistently, any previous history of STIs, age, and more contribute to your risk as well.

Risk factors of STIs
Verywell / Theresa Chiechi​

Common Causes

The Centers for Disease Control and Prevention (CDC) reports that nearly 2.5 million cases of sexually transmitted infections are reported each year in the United States.

There are a number of different STIs, such as human papillomavirus (HPV), herpes, chlamydia, gonorrhea, and HIV. Some STIs are due to viruses, while others are due to bacteria.

Depending on the infection, they can spread through body fluids including blood, saliva, semen, or vaginal secretions, or be transmitted by direct skin-to-skin contact. This primarily occurs with sexual contact. However, people who are pregnant can also transmit some STIs via vertical transmission or through breast milk.


You may also hear the term sexually transmitted infection (STI). Technically, there is a difference between the two. STIs are infections that cause STDs. For example, chlamydia (an STD) is caused by a sexually transmitted Chlamydia trachomatis bacterial infection. However, in the context of your risk, you can consider the two terms interchangeable.

Consistent use of external or internal condoms and other barriers can prevent STIs transmitted through body fluids, such as HIV and chlamydia. But it may not offer protection against herpes and other diseases spread through skin-to-skin contact.

Your odds of contracting an STI depends on a number of factors, including:

  • How you have sex (manual, anal, vaginal, oral)
  • How many partners you have
  • What type of encounters you have
  • Whether you practice safer sex
  • How consistently you use external or internal condoms or other barriers
  • If you use barriers for intercourse only or oral sex as well
  • Whether you use lubricants and what kinds you use (some, for example, can degrade latex external condoms)
  • Whether your partner has an STI and, if so, what type
  • The severity of your partner's infection (as measured by viral load and other factors)
  • Whether you have breaks in your skin, infections, or other STIs that make you more susceptible to infection
  • Your overall health and the health of your immune system

Rates of STIs are on the rise, the CDC reports. Between 2014 and 2018, cases of syphilis increased by 71%, gonorrhea by 63%, and chlamydia by 14%. New cases of HIV, however, have declined, with nearly 38,000 new cases reported in 2018.

Lifestyle Risk Factors

There are many things that you can do to protect yourself against STIs. By being aware of the main risk factors that you can control, it's possible to stay healthy without being abstinent.

Here are common lifestyle risk factors for STIs and what you should know about each. 


People who are under age 25 are far more likely to be infected with STIs than older people for several reasons.

First, young women are more biologically susceptible to STIs than older women. Their bodies are smaller and they are more likely to experience tearing during intercourse. Their cervixes also aren't fully developed and are more susceptible to infection by chlamydia, gonorrhea, and other STIs.

Finally, in general, young people are more likely to engage in sexual risk-taking and are more likely to have multiple partners.


Gay and bisexual men, or other men who have sex with men (MSM), are disproportionately impacted by syphilis, HIV, and other STIs.

In 2018, nearly half of reported syphilis cases in both men and women were traced to men who have sex with only men, the CDC reports.

According to a study published in the American Journal of Public Health, unprotected anal intercourse (for both men and women) increases the likelihood of contracting an STI because of the rigidity and fragility of rectal tissue. This makes anal tissue more susceptible to tearing, upping the risk of becoming infected. 

Unprotected Sex

Although using a condom or other barrier method of birth control isn't a guarantee you won't become infected with a STI, it's a highly effective way to protect yourself.

Even viruses like HPV, which external and internal condoms are less effective against, have reduced transmission rates when condoms are used.

Other than abstinence, consistent condom use—which means using an external or internal condom every time you have sex—is the best way to prevent STIs.

This applies even if you are using birth control such as the pill or an intrauterine device (IUD). Once protected from pregnancy, some people are reluctant to use condoms as part of their sexual routine.

Prescription birth control does not protect you from STIs. Dual protection with the additional use of condoms is best.

A History of STIs

Having one STI frequently makes you more susceptible to infection by other STIs. It's easier for another pathogen to infect tissue that is already irritated, inflamed, or blistered.

Having an STI is also an indirect reflection of your risk of new infection: Since you were exposed once already, it suggests that other factors in your lifestyle may be putting you at risk too.

Multiple Partners

The more partners you have, the more likely it is that you will be exposed to an STI. Furthermore, people with multiple partners tend to have partners with multiple partners.

Serial Monogamy

Some people only date one person at a time but still date a large number of people each year. This is referred to as serial monogamy.

The danger for people who practice serial monogamy is that each time they are involved in an "exclusive" sexual relationship, they are likely to be tempted to stop using safer sex precautions.

But monogamy is only an effective way to prevent STIs in long-term relationships when both of you have tested negative.

Additionally, some tests aren't reliable until you've been living with the STI for some time. Unfortunately, many serially monogamous relationships don't last long enough for that to be a viable option.

Alcohol Use

Drinking can be bad for your sexual health in many different ways. People who use alcohol on a regular basis, particularly in social situations, may be less discriminating about whom they choose to have sex with.

Alcohol also lowers inhibitions. It may also make it more difficult to convince a sexual partner to use an external or internal condom or to use one correctly.

Recreational Drug Use

People who have sex under the influence of drugs are more likely to engage in risky sexual behaviors, such as having condomless sex or sex without other forms of protection.

Drugs may also make it easier for someone to pressure you into engaging in sexual behaviors. Injection drug use, in particular, is associated with an increased risk of blood-borne diseases such as HIV and hepatitis.

Trading Sex for Money or Drugs

People who trade sex for money or drugs may not be sufficiently empowered to negotiate safer sex. And partners acquired in this manner are far more likely to be living with an STI than people in the general population.

Note: Some sex workers, particularly those who have made an independent and informed choice to engage in their work, are highly conscientious about safer sex and prevention. Risk varies according to individual behaviors, just as it does for people who don't engage in commercial sex.


7 Tips For Preventing STDs

A Word From Verywell

STIs are largely preventable. While abstaining from all sexual contact is the only way to completely prevent getting a sexually transmitted infection, only having sex in a mutually monogamous relationship can also improve your odds of not acquiring an STI. In addition, practicing safer sex every time you engage in sexual activity can dramatically decrease your risk of contracting an STI.

Frequently Asked Questions

  • How common are STIs?

    STIs are very common, resulting in millions of new infections every year worldwide. In fact, according to 2018 data from the CDC, 1 in 5 people in the U.S. had an STI. The most common ones are chlamydia, gonorrhea, syphilis, and trichomoniasis.

  • What symptoms appear with STIs?

    Some STIs show no symptoms, which is why prevention and testing are important. If an STI causes symptoms, they are typically unusual discharge, sores or warts on the infected area, frequent urination, burning sensation when urinating, itching and redness, abnormal odor in the genital area, abdominal pain, and fever.

15 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

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.