Bacteriology Histopathology of Treponema pallidumalt Treponema pallidum subspecies pallidum is a spiral-shaped, Gram-negative, highly mobile bacterium. Three other human diseases are caused by related Treponema pallidum subspecies, including yaws (subspecies pertenue), pinta (subspecies carateum) and bejel (subspecies endemicum). Unlike subspecies pallidum, they do not cause neurological disease. Humans are the only known natural reservoir for subspecies pallidum. It is unable to survive more than a few days without a host. This is due to its small genome (1.14Mbp) failing to encode the metabolic pathways necessary to make most of its macronutrients. It has a slow doubling time of greater than 30 hours. The bacterium is known for its ability to evade the immune system and its invasiveness. Transmission<unk>  is transmitted primarily by sexual contact or during pregnancy from a mother to her baby; the spirochete is able to pass through intact mucous membranes or compromised skin. It is thus transmissible by kissing near a lesion, as well as oral, vaginal, and anal sex. Approximately 30% to 60% of those exposed to primary or secondary<unk>  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 new cases in the United States (60%) occur in men who have sex with men; and in this population 20% of<unk>  cases were due to oral sex alone.<unk>  can be transmitted by blood products, but the risk is low due to screening of donated blood in many countries. The risk of transmission from sharing needles appears to be limited. It is not generally possible to contract<unk>  through toilet seats, daily activities, hot tubs, or sharing eating utensils or clothing. This is mainly because the bacteria die very quickly outside of the body, making transmission by objects extremely difficult.
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