Treponema pallidum subspecies pallidum—the cause of syphilis—is a thin, spiral organism with a cell body surrounded by a trilaminar cytoplasmic membrane. Humans are the only natural host, and the organism cannot be cultured in vitro.

• Cases are acquired by sexual contact with infectious lesions (chancre, mucous patch, skin rash, condyloma latum); nonsexual acquisition through close personal contact, infection in utero, blood transfusion, and organ transplantation is less common.
• There are ~12 million new infections per year worldwide.
  • - In the United States, 31,575 cases were reported in 2000.
  • - The reported cases of primary and secondary syphilis combined (which better indicate disease activity) increased from <6000 in 2000 to >14,000 in 2012, primarily affecting MSM, 20-70% of whom were co-infected with HIV.
• One-third to one-half of sexual contacts of persons with infectious syphilis become infected—a figure that underscores the importance of treating all recently exposed sexual contacts.