Quick Answer
The etiologic agent of syphilis is the bacterium Treponema pallidum subspecies pallidum. It is a spirochete, a thin spiral-shaped bacterium that penetrates mucous membranes or breaks in the skin during sexual contact and then spreads through the bloodstream. Syphilis is classified as a bacterial sexually transmitted infection, and it is curable with antibiotic treatment.
- Treponema pallidum is a spirochete, a spiral-shaped bacterium with a corkscrew-like motion.
- Syphilis is classified as a bacterial sexually transmitted infection, not a viral or fungal infection.
- The bacterium is fragile outside the human body and does not survive long on surfaces.
- Transmission usually requires direct contact with an infectious lesion or mucous membrane.
- Infection is staged as primary, secondary, latent, or tertiary based on signs and timing.
- Benzathine penicillin is the preferred treatment for most stages, per CDC guidance.
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Knowing the exact etiologic agent matters because it explains how syphilis spreads, why it can affect nearly any organ, and why staging guides treatment. Treponema pallidum is unusually thin, so routine light microscopy often misses it; diagnosis relies on blood tests, darkfield examination of lesion fluid in some settings, and sometimes molecular testing. Once inside, the bacterium multiplies locally, then enters lymphatics and blood, producing the classic stages. Because the organism cannot be cultured easily in standard labs, clinicians classify syphilis by clinical stage and serology rather than by isolating the bacterium.
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Sources & Further Reading
- CDC — Sexually Transmitted Infections
- CDC STI Treatment Guidelines
- WHO — Sexually transmitted infections fact sheet
- MedlinePlus — Sexually Transmitted Diseases
Figures reflect CDC surveillance and WHO estimates; they are population statistics, not personal risk predictions.