Causes and Risk Factors of Syphilis

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

There are certain risk factors that increase your chances of contracting syphilis. Read on to learn more about them and how the bacterium is passed from one person to the next so you can take precautions to protect yourself.

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

Common Causes

Sexual contact is the main way that people get syphilis. A pregnant mother can also pass it to her baby.

Sexual Contact

Sexual transmission of syphilis infection occurs when skin or mucosal tissue comes in 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.

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 it's not treated, syphilis will go through four stages of infection: primary, secondary, 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 sores that may be firm round or painless.
  • During secondary syphilis, the disease can be passed by coming into contact with the secondary rash.
  • During latent syphilis, there are no signs of symptoms and the infection generally can not be spread.
  • During tertiary syphilis, the disease has spread to other organs and is likely highly contagious at this point.

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

Mother to Child

The perinatal transmission of syphilis, also known as congenital syphilis, occurs when the syphilis bacteria in a pregnant mother penetrates the placenta surrounding the developing fetus.

While this can happen during any stage of pregnancy, the likelihood is highest during the second half. The risk of transmission varies by the mother's stage of infection.

According to the Centers for Disease Control and Prevention (CDC), around 40% of babies born to women with untreated syphilis can be stillborn or die from complications of the infection shortly after birth.

Lifestyle Risk Factors

While syphilis can affect anyone, there are a number of risk factors that can increase your likelihood of infection.

Among the most common risk factors are:

  • Inconsistent condom use: This is the main cause of transmission in all groups. According to a study by the CDC, only around 24% of females and 33% of males between the ages 15 and 44 use a condom consistently.
  • Multiple sex partners: This places you at risk due to increased chances of exposure. This is especially true among anonymous partners who meet on online platforms.
  • Gay, bisexual, and other men who have sex with men (MSM): These individuals account for approximately half of primary and secondary 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 STI outbreaks. Although syphilis is rarely 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.

Avoidance of Screening

Perhaps one of the greatest obstacles to detecting and treating syphilis infection is avoidance of STI screening. This can be for a variety of reasons.

Some people may not get tested due to lack of access to health care, while others may be unaware of the screening guidelines.

Others may actively ignore them. Reasons for avoiding tests can include stigma or the fear of receiving a concurrent HIV diagnosis. This translates to an increased risk of infection, as well as reinfection.

​​A 2015 study from the University of California, Los Angeles reported that between 6% and 8% 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.

Young African American males are 62% less likely to be tested if they associate STIs with immorality, shame, uncleanliness, or a weakness of character. Today, the rate of syphilis among African Americans is nearly five times that of whites.

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 2.2 cases per 100,000 people). By 2019, that number had increased to nearly 39,000 (or 12 cases per 100,000 people).

Testing Guidelines

The CDC now recommends at least once-a-year testing for syphilis, chlamydia, and gonorrhea for all sexually active gay men, bisexual men, and other men who have sex with men.

All sexually active persons with HIV should also be screened for these STIs at their initial HIV care visit and at least annually during the course of their care.

Additionally, pregnant women should be tested for syphilis at their first prenatal visit.

All sexually active gay and bisexual men who have multiple or anonymous sex partners should be screened more often (e.g., at three- to six-month intervals).

Not adhering to these guidelines can increase your chances of having an undetected case of syphilis that then gets passed on to partners.

A Word From Verywell

When it comes to your health, be honest with yourself about your risk and do what you can to protect yourself and anyone who you could potentially expose to infection.

If expense is making you hesitate getting tested, look into some of the ways to get tested for STIs for free or at reduced cost.

Frequently Asked Questions

  • 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 or sore. 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, because the bacteria quickly die once it is outside the body.

  • What causes neurosyphilis?

    Neurosyphilis is a complication of untreated syphilis that can occur up to 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 people are routinely screened in most U.S. states to ensure early treatment and a reduction in the risk of transmission or complications.

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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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.