Syphilis is caused by the bacterium Treponema pallidum.
Transmission of the bacteria usually occurs during vaginal, anal, or oral sex. The syphilis bacteria are passed from person to person through direct contact with:
Sores mainly occur on the external genitals, vagina, anus, or rectum. Sores can also occur on the lips and in or around the mouth. The bacteria most commonly enter the body through mucous membranes, usually in the area around the genitals and urinary system.
In rare cases, syphilis enters the body through openings in the skin, such as cuts and scrapes, or even through wet kisses, if the infected person has a sore on the mouth or lips. Syphilis may also be transmitted by using a needle previously used by an infected person. Syphilis can be transmitted through a blood transfusion. But this is very rare, because all donated blood in the United States and Canada is screened for some sexually transmitted infections (STIs). And syphilis bacteria cannot survive more than 24 to 48 hours in blood stored using modern blood-banking methods.
A pregnant woman with syphilis can pass the infection through the placenta and infect her baby any time during pregnancy or delivery (congenital syphilis).
Syphilis cannot be spread through casual contact with toilet seats, door knobs, swimming pools, hot tubs, bathtubs, shared clothing, or eating utensils.
Having been infected with syphilis in the past does not protect a person from becoming infected again.
An incubation period is the time between exposure to a disease and the first symptom. A skin sore called a chancre is usually the first symptom of sexually transmitted syphilis. A chancre appears between 3 weeks to 3 months after a person has been infected with syphilis.
A person with syphilis can easily pass the infection (is contagious) to physically intimate partners when primary- or secondary-stage sores are present. But the person may be contagious for years, off and on, and is always contagious whenever an open sore or skin rash from syphilis is present.