Syphilis is an infectious disease, usually spread through sexual contact. Symptoms of the disease appear in readily identifiable stages, but not all people progress through each stage. This is true even if the disease is left untreated. Syphilis can take years to move through these stages, with up to 20 or 30 years passing until the late stages of the disease.

Syphilis is divided into three stages – primary, secondary and tertiary – with a lengthy latent period between the last two. Primary and secondary syphilis occur within a year of initial infection.

Syphilis spreads when the bacteria (called Treponema pallidum) in a syphilis sore (chancre) comes into contact with an open cut, lesion or opening in a mucous membrane. This usually occurs in the genital areas during sexual contact. Syphilis can spread through vaginal, anal and oral sex. Close contact such as kissing may also spread the disease, if sores or cuts are present in the mouth. Injecting intravenous drugs with an infected needle can also transmit syphilis. The syphilis bacteria cannot be spread through pools, hot tubs, doorknobs, toilet seats or other inanimate objects.

After infection, the primary stage of syphilis (usually a single chancre at the infection site) may not even be noticeable. In its secondary stage, syphilis presents symptoms that are common with various other diseases (e.g., rash, fever, weight loss). In both stages, the symptoms resolve without treatment. However, even when symptoms appear to be healing, people can still spread the disease.

After its secondary stage, syphilis may enter a long latent period, when patients have no symptoms. The final tertiary stage may begin many years later and affect the heart, brain, spinal cord or skin. Many people with syphilis never progress to the secondary or tertiary stages.

The complications of syphilis are associated with tertiary syphilis. They can affect many body systems and if left untreated, eventually cause death. Syphilis in every stage also affects pregnant women, who can pass the disease to their fetus. In addition to complications (e.g., vision and hearing loss) for children born to mothers with syphilis, the disease may also cause miscarriages or stillbirths.

Like other sexually transmitted diseases, infection with syphilis may also facilitate transmission of human immunodeficiency virus (HIV). The Centers for Disease Control and Prevention (CDC) estimates that individuals with syphilis have between two to five times greater risk of acquiring HIV than people without syphilis.

Reports of primary and secondary syphilis cases in the United States decreased by almost 90 percent throughout the 1990s and into 2000, to the lowest number since the CDC began recording the disease in 1941. According to the CDC, 33,401 new cases of syphilis were reported in 2004. From 2000 to 2005, the overall increase in primary and secondary syphilis cases was observed mainly among men. Some of that increase may be attributed to reported outbreaks of syphilis among men who have sex with men. However, syphilis rates for women, which were previously declining, increased 12.5 percent in that time period (from 0.8 cases to 0.9 cases for every 100,000 women).