Syphilis

Syphilis is a sexually transmitted infection caused by the spirochete *Treponema pallidum*. After entering through sexual contact, the organism triggers both humoral and cellular immune responses. The visible lesion is not simply “a sore”; it reflects local infection and inflammation, while serologic changes reflect the systemic immune response. Primary syphilis classically presents with a chancre: a painless ulcer at the site of exposure. A painless genital, anal, or oral ulcer deserves testing rather than reassurance. The lesion may heal without treatment, but healing does not mean the infection has cleared. As infection becomes systemic, secondary syphilis can produce a diffuse rash, sometimes involving the palms and soles, along with generalized symptoms. Latent syphilis has no visible findings, which is why a sexual history, prior testing, and serology matter even when the skin examination is normal. Stage determines both treatment duration and the timing of follow-up.