Which bacterium causes syphilis, the sexually transmitted infection described by its distinctive stages?

Treponema pallidum causes syphilis, the sexually transmitted infection described by its distinctive stages.

This slender, spiral-shaped bacterium is a spirochete. It spreads mainly through direct contact with syphilitic sores, although infection can also pass from a pregnant person to a fetus. Syphilis commonly progresses through primary, secondary, latent, and tertiary stages.

The bacterium is difficult to grow in ordinary laboratory culture, so diagnosis commonly uses blood tests or microscopic and molecular methods. Penicillin remains the standard treatment for most cases.

Treponema pallidum is often confused with Neisseria gonorrhoeae, the bacterium that causes gonorrhea. The two infections can occur together, but they are different diseases caused by different organisms.