性传播疾病梅毒患者病损硬下疳中可见梅毒螺旋体。病原体为苍白密螺旋体。晚期妊娠患者可传播给胎儿，从而使新生儿患先天性梅毒。病变组织学特征为动脉内膜炎和浆细胞浸润性炎。早期梅毒的重要特征是感染梅毒螺旋体两周后在原始部位形成硬下疳。自愈后发生二期梅毒，持续两个月，其特征性病变是梅毒疹，随后进入静止期。但经十几年的发展可出现三期梅毒的典型病损如神经系统梅毒、心血管梅毒或梅毒瘤。此期很难查见病原体。
Seen here are spirochetes from a lesion of primary syphilis known a chancre. The causative organism is Treponema pallidum. This is a sexually transmitted disease (STD). Mothers who are infected may pass the spirochetes to fetuses in the third trimester, resulting in congenital syphilis. The characteristic histopathologic findings include endarteritis and inflammatory infiltrates with numerous plasma cells. The primary stage of syphilis is evident a couple of weeks after initial infection as a chancre at the site of inoculation. This spontaneously heals, and secondary syphilis ensues, lasting for a couple of months, characterized by a rash. The disease then becomes quiescent, but manifestations of tertiary syphilis may develop decades later as neurosyphilis, cardiovascular syphilis, or gummatous necrosis in visceral organs or soft tissues. In this tertiary stage, the spirochetes are difficult to demonstrate in tissues.