Causes and Risk Factors of Syphilis

Syphilis is sexually transmitted infection (STI) caused by a spiral-shaped bacterium called Treponema pallidum. The infection is typically transmitted through contact with a sore during a sexual encounter with an infected person, although a mother can pass to her child during pregnancy.

People tend to have misconceptions about the causes and risk factors associated with syphilis. Learn more about how to protect yourself from this STI and treatment options available in case of infection.

syphilis causes
Illustration by Joshua Seong. © Verywell, 2018.

Common Causes

Anyone with syphilis either contracted the infection from sexual contact or from their mother in utero.

Adult Transmission

Syphilis infection occurs when skin or mucosal tissue comes into contact with an open, ulcerative sore known as a chancre. The corkscrew shape of the bacteria allows it to burrow into the mucous membranes of the mouth, vagina, or rectum or to enter microscopic breaks in the skin.

Syphilis Through Sexual Encounter

In adults and sexually active youth, syphilis is passed almost exclusively through oral, vaginal, or anal sex. The infection can also pass from one person to another through kissing if one partner has a sore in their mouth - though this form of transmission is not nearly as common.

If left untreated, syphilis will go through five stages of infection: primary, secondary, early latent, late latent, and tertiary. The risk and mode of transmission can vary by the stage:

  • During primary syphilis, the disease is passed by coming into contact with a chancre.
  • During secondary syphilis, the disease can be passed by coming into contact with the secondary rash.
  • During early latent syphilis, secondary symptoms can sometimes relapse and increase the risk of transmission.
  • During late latent or tertiary syphilis, the disease is considered non-contagious.

Syphilis cannot be passed via toilet seats, casual contact, or the shared use of utensils or personal care items. This is because T. pallidum has a fragile shell that lacks the lipoproteins needed to sustain life outside of the body for very long.

Transmission by Objects Very Rare

As a result, the object-to-human transmission of syphilis is extremely unlikely.

Perinatal Transmission

The perinatal transmission of syphilis (also known as congenital syphilis) occurs when the syphilis bacteria penetrates the placenta surrounding the developing fetus. While this can happen during any stage of the pregnancy, the likelihood is greatest during the second half. The risk of transmission varies by the mother's stage of infection.

Mother-to-Baby Transmission

The risk of transmission of syphilis from a mother who is untreated is very high.

Lifestyle Risk Factors

While syphilis can affect anyone, there are a number of risk factors that can increase your likelihood of infection. Some are related to sexual behaviors, while others are associated with characteristics that can place an entire population at risk.

Among the most common risk factors:

  • Inconsistent condom use is the main cause of transmission in all groups. According to a study by the Centers for Disease Control and Prevention (CDC), only around 24 percent of women and 33 percent of men between the ages 15 and 44 use a condom consistently.
  • Multiple sex partners place you at risk simply by providing you a greater opportunity for exposure. This is especially true among anonymous partners who meet on online platforms.
  • Men who have sex with men (MSM) account for approximately 60 percent of syphilis infections in the United States. Physiological vulnerabilities (such as the fragility of rectal tissues) and high rates of HIV place MSM at an inherently increased risk compared to heterosexual peers.
  • Injecting drug use has given rise to a spate of STD outbreaks in recent years (including the 2015 HIV outbreak in Indiana linked to oxycontin). While syphilis is not transmitted by blood-to-blood exposure, injecting drug use can impair judgment and increase the risk of sexual violence or the exchange of sex for drugs.

Risk Attitudes and Behaviors

Perhaps one of the greatest risks factors of syphilis infection is the avoidance of STD screening. 

STI Testing Recommendations

While it was once thought that the need for screening depended on the number of partners someone had, the CDC now recommends at least once-a-year testing (syphilis, chlamydia, and gonorrhea) for all sexually active gay men, bisexual men, and other men who have sex with men.

Those who have multiple or anonymous sex partners should be screened more often (e.g., at three- to six-month intervals).

Unfortunately, many of these men are either unaware of the guidelines or actively ignore them, either because of stigma or the fear of receiving a concurrent HIV diagnosis. This translates to not only an increased risk of infection but reinfection.

​​A 2015 study from the University of California, Los Angeles reported that between 6 percent and 8 percent of MSM previously infected with syphilis experienced a repeat infection within two years. Many of those who had delayed testing until secondary infection admitted that they either didn't want to know or were afraid to learn the results.

The same has been seen in young African American men who are 62 percent less likely to be tested if they associate STDs with immorality, shame, uncleanliness, or a weakness of character. Today, the rate of syphilis among African Americans is nearly five times that of the white population, largely as a result of these attitudes.

Syphilis Infections On the Rise

Syphilis infections in the United States are increasing steadily in many ethnic and age groups.

In 2000, fewer than 6,000 cases of primary or secondary syphilis were reported (or 2.1 cases per 100,000 people); by 2017, that number had increased to over 30,000 (or 9.5 cases per 100,000).

Until these attitudes and testing behaviors are changed, the general risk of infection can be expected to rise. Be honest with yourself about your risk and do what you can to protect yourself.

Frequently Asked Questions

  • What causes syphilis?

    Syphilis is caused by a spiral-shaped bacteria known as Treponema pallidum. The bacteria is highly transmissible as it is able to penetrate intact mucous membranes and establish an infection with only a small number of bacteria.

  • How is syphilis transmitted?

    Syphilis is primarily a sexually transmitted infection that causes a painless ulcerative sore known as a chancre.  Syphilis is spread when mucous membranes or non-intact skin comes into contact with a chancre. Syphilis can also be passed from mother to child during pregnancy.

  • How is syphilis not transmitted?

    Syphilis cannot be passed through casual contact, toilet seats, shared utensils, or touching objects, as the bacteria quickly dies once it is outside the body. Transmission through blood transfusions is also rare given that the blood supply is routinely screened in the United States. Syphilis is also not associated with injecting drug use.

  • What are the risk factors for syphilis?

    The main risk factors for syphilis include:

    • Condomless sex (including vaginal, anal, and oral sex)
    • Multiple sexual partners
    • Men who have sex with men (who account for the lion's share of infections)
  • What causes neurosyphilis?

    Neurosyphilis is a complication of untreated syphilis that occurs 10 to 20 years after the initial (primary) infection. Roughly 10% of untreated individuals will go on to develop neurosyphilis. People with HIV account for the majority of cases.

  • What causes congenital syphilis?

    Congenital syphilis is caused when a mother with syphilis passes the infection to her unborn baby. It can occur at any stage of pregnancy. Pregnant persons are routinely screened in most U.S. states to ensure early treatment and a reduction in the risk of transmission or complications.

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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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