Image of syphilis bacteria

Treponema pallidum, the bacteria that cause syphilis.

Credit: NIAID

Syphilis is a sexually transmitted infection (STI) caused by the bacterium Treponema pallidum. It is transmitted from person to person via direct contact with a syphilitic sore, known as a chancre. These sores can be found on the genitals, vagina, anus, rectum, lips and mouth. Pregnant women can transmit the infection to their unborn child.  Syphilis can cause long-term health complications if left untreated.

Why Is the Study of Syphilis a Priority for NIAID?

An ancient bacterium, syphilis is still of major importance today. The Centers for Disease Control and Prevention estimates that each year 55,400 people in the United States get new syphilis infections. During 2012, there were 49,903 reported new cases. In addition, HIV infection and syphilis are linked. Syphilis increases the risk of transmitting as well as acquiring HIV.

How Is NIAID Addressing This Critical Topic?

Developing better ways to diagnose and treat syphilis is an important research goal of scientists supported by NIAID.

Scientists are developing new tests that may provide better ways to diagnose syphilis and define the stage of infection. Efforts to develop a diagnostic test that would not require a blood sample are a high priority. For example, researchers are evaluating tests using saliva and urine to see whether they would work as well as blood tests. Researchers are also trying to develop other diagnostic tests for detecting infection in babies.


NIAID-funded researchers have sequenced the genetic blueprint, or genome, of the bacterium that causes syphilis. The DNA sequence represents an encyclopedia of information about the bacterium. Researchers have identified clues in the genome that may help better diagnose, treat, and vaccinate against syphilis.


In an effort to stem the spread of syphilis, scientists are conducting research that could lead to the development of a vaccine. Molecular biologists are learning more about the various surface parts of the syphilis bacterium which stimulate the immune system to respond to it.


Another research priority is to develop a safe, effective, single-dose oral antibiotic for syphilis. In 2010, a clinical trial found that antibiotic pills (azithromycin) are as effective as penicillin injections for curing early-stage syphilis. Azithromycin may give healthcare providers a good alternative for treating HIV-negative people with early-stage syphilis. This is especially important in settings where resources are limited because penicillin injections can cause allergic reactions and the drug must be refrigerated and given by trained personnel.

Information for Researchers

NIAID offers resources, information about funding, and ways to connect to other scientists researching sexually transmitted diseases including syphilis.

Read more information for researchers of STIs

Related Public Health and Government Information

To learn about risk factors for smallpox and current prevention and treatment strategies visit the MedlinePlus syphilis site.

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