Syphilis is an infection caused by the spiral-shaped bacterium Treponema pallidum. While the disease is primarily known for transmission through intimate contact, this is not the only route of infection. The bacterium spreads when a person comes into direct physical contact with an active, infectious sore. Although the majority of cases occur through sexual activity, non-sexual transmission is medically confirmed.
Understanding How Syphilis Is Transmitted
The mechanism of syphilis transmission requires direct contact with an open lesion, known as a chancre, or a secondary-stage rash. These infectious sores typically appear during the primary and secondary stages of the disease, which are the most contagious phases. The bacteria enter the body through minor cuts, abrasions, or the moist lining of mucous membranes, such as those found in the mouth or genitals.
A chancre is a firm, painless ulcer that develops at the site where the bacteria first entered the body, usually appearing about three weeks after initial exposure. During the secondary stage, a non-itchy rash often appears, which can cover the body, frequently involving the palms of the hands and the soles of the feet. This rash and any moist, wart-like patches (condylomata lata) that may develop are also highly infectious.
Sexual activity remains the most frequent way the disease spreads because it involves direct, prolonged skin-to-skin contact, often near active lesions that may be unnoticed. Friction during these activities increases the likelihood of the bacteria penetrating the skin or mucosal barrier. The infection can be transmitted through vaginal, anal, or oral contact if a sore is present in the mouth, on the lips, or in the genital or anal area.
Non-Sexual Ways Syphilis Can Spread
The most medically significant non-sexual route of transmission is congenital syphilis, which occurs when the infection is passed from an infected pregnant person to the fetus. The Treponema pallidum bacteria can cross the placental barrier, infecting the developing baby at any point during pregnancy. This transplacental spread is particularly likely if the mother has primary or secondary syphilis, with transmission rates reaching between 50% and 70% in these early stages.
This transfer can lead to severe outcomes, including miscarriage, premature birth, stillbirth, and serious, lifelong health complications for the infant. Nearly all pregnant individuals with untreated early syphilis will experience an adverse fetal outcome, demonstrating the severity of this non-sexual pathway. Routine screening and early treatment with penicillin during pregnancy are highly effective measures to prevent congenital infection.
Transmission through contaminated blood products is another confirmed, non-sexual route, though it is exceedingly rare today. Rigorous blood donor screening processes and the standard refrigeration methods used for blood storage effectively kill the fragile Treponema pallidum bacterium. While technically possible, the risk of acquiring syphilis from a blood transfusion in modern healthcare settings is extremely low.
Rare instances of direct non-sexual contact transmission are also documented. This can happen if an open cut or abrasion on one person’s skin comes into contact with an active, weeping chancre on a non-genital area, such as a sore on the hand or mouth. This form of spread requires prolonged contact with the lesion itself, not just casual touching.
Dispelling Transmission Misconceptions
A frequent source of public concern involves the possibility of contracting syphilis from inanimate objects, known as fomites, or through casual daily interactions. The scientific reality is that the bacterium Treponema pallidum is extremely delicate and cannot survive for long outside of the human body. It is highly susceptible to drying, air exposure, and common environmental changes.
This biological limitation means that transmission is virtually impossible from surfaces such as toilet seats, doorknobs, bathtubs, or shared eating utensils. The bacteria are quickly inactivated once exposed to the open air. Therefore, acquiring the infection through casual contact in public or shared spaces is not a concern.
Casual, brief touching of an infected person’s skin that does not involve an active sore poses no risk of transmission. Simple activities, such as hugging or sharing towels, do not facilitate the necessary direct contact with an infectious lesion required for spread.
The risk associated with kissing merits specific clarification, as it is a form of direct contact. Casual, closed-mouth kissing is considered very low risk. Deep or “French” kissing carries a minimal risk only if one person has an active, open chancre on the lips or inside the mouth. Since these chancres are often painless and may go unnoticed, the potential for transmission exists, though it is far less common than genital spread.