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 before the last stage of the disease develops.

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 area during sexual contact. Although syphilis bacteria multiply at a relatively slow rate, these bacteria may simultaneously spread throughout the body via the lymphatic system. Because of this, symptoms of syphilis can appear in several places in the body and are often confused with other conditions.  

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 survive on inanimate objects and so the disease cannot be spread via pools, hot tubs, doorknobs, toilet seats or other such objects.

After infection, the primary stage of syphilis (usually a single sore 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 may resolve without treatment. However, even when symptoms appear to be healing, people are still infectious and can 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 the tertiary stage of infection. Many body systems may be affected and, if the syphilis is left untreated, it may eventually cause death. Syphilis at every stage can affect pregnant women, who may pass the disease to their fetus. Syphilis can also cause miscarriage or stillbirth.

People with human immunodeficiency virus (HIV) and the associated acquired immune deficiency syndrome (AIDS) also experience complications with syphilis. HIV-positive patients are more likely to be infected with syphilis. Also, people with syphilis are more likely to become infected with HIV due to the presence of skin lesions, which can allow the virus entry into the body. Additionally, people with HIV or AIDS are more likely to experience the neurological symptoms associated with tertiary syphilis at an earlier stage than people without HIV or AIDS. 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, more than 34,000 new cases of syphilis were reported in 2003. From 2000 to 2004, syphilis cases increased, but only among men. Some of that increase may be attributed to reported outbreaks of syphilis among men who have sex with men. Syphilis rates for women continued to decline in that time period. Syphilis rates among African-Americans declined as well; however, this population remains the most affected by the disease.