Syphilis is a sexually transmitted infection (STI) that can lead to serious health issues affecting the brain, nervous system, eyes, or ears if left untreated. It progresses through four stages (primary, secondary, latent, and tertiary), each with its own symptoms. Syphilis is transmitted through direct contact with a syphilis sore during vaginal, anal, or oral sex. It can also pass from a mother with syphilis to her unborn baby. Treatments for syphilis include: