Syphilis is a sexually transmitted infection caused by the bacterium Treponema pallidum. It progresses through several stages, including primary, secondary, latent, and tertiary syphilis, each with distinct symptoms. Early symptoms may include sores, rashes, and flu-like symptoms, while untreated syphilis can lead to severe health complications affecting the heart, brain, and other organs. Diagnosis and treatment typically involve blood tests and antibiotics, usually penicillin.