Step 1: Understanding syphilis.
Syphilis is a sexually transmitted disease characterized by ulcers, rashes, and systemic complications if untreated.
Step 2: Causative organism.
The disease is caused by a spiral-shaped bacterium known as {Treponema pallidum}.
Step 3: Eliminating other options.
{Neisseria gonorrhoeae} is a diplococcus causing gonorrhoea, not syphilis.
Step 4: Conclusion.
Therefore, syphilis is caused by {Treponema pallidum}.