Syphilis is a sexually transmitted disease caused by the bacteria Treponema pallidum. It progresses through three stages - primary, secondary, and tertiary syphilis. Primary syphilis involves skin sores called chancres. Secondary syphilis occurs after chancres heal and involves rashes and mucous membrane lesions. Tertiary syphilis develops years later and can affect the heart, blood vessels, or brain. Syphilis can also be transmitted from mother to child during pregnancy. Diagnosis involves dark-field microscopy of sores or serological tests detecting antibodies produced against T. pallidum.