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Why can treatment stop syphilis but not repair damage from late-stage disease?

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Scanning electron micrograph of Treponema pallidum, the spirochete bacterium that is the causative agent of syphilis. — syphilis Scanning electron micrograph of *Treponema pallidum*, the spirochete bacterium that is the causative agent of syphilis.

Syphilis is a systemic disease caused by the bacterium Treponema pallidum. Antibiotics, such as penicillin, are highly effective at killing these spirochetes and halting the progression of the infection at any stage.123

However, treatment cannot repair damage that has already occurred during the late, or tertiary, stage of the disease. By this stage, the bacteria have often caused extensive, permanent destruction to the body's tissues and organs. For example, in cardiovascular syphilis, the spirochetes destroy the elastic tissue of the aorta, leading to irreversible structural changes like aneurysms. Similarly, in neurosyphilis, the bacteria cause widespread, permanent degeneration of the brain or spinal cord, resulting in lasting neurological deficits. Because these conditions involve the physical destruction of vital biological structures, eliminating the bacteria stops the disease from worsening but cannot restore the damaged anatomy or function.34