Bacteriology
Treponema pallidum subspecies pallidum is a spiral-shaped, Gram-negative, highly mobile bacterium. Three other human diseases are caused by related Treponema pallidum, including yaws (subspecies pertenue), pinta (subspecies carateum) and bejel (subspecies endemicum). Unlike subtype pallidum they do not cause neurological disease. Humans are the only known natural reservoir for subspecies pallidum. It is unable to survive without a host for more than a few days. This is due to its small genome (1.14 MDa) and thus its inability to make most of its macronutrients. It has a slow doubling time of greater than 30 hours.
Transmission
Syphilis is transmitted primarily by sexual contact or during pregnancy from a mother to her fetus; the spirochaete is able to pass through intact mucous membranes or compromised skin. It is thus transmissible by kissing, oral, vaginal, and anal sex. Approximately 30 to 60% of those exposed to primary or secondary syphilis will get the disease. Its infectivity is exemplified by the fact that an individual inoculated with only 57 organisms has a 50% chance of being infected. Most (60%) of new cases in the United States occur in men who have sex with men. It can be transmitted via blood products. However, it is tested for in many countries and thus the risk is low. The risk of transmission from sharing needles appears limited. Syphilis cannot be contracted through toilet seats, daily activities, hot tubs, or sharing eating utensils or clothing.