Syphilis is a sexually transmitted disease caused by the bacterium Treponema pallidum. It has various stages including primary, secondary, latent, and tertiary syphilis. Primary syphilis involves a painless sore or ulcer called a chancre, usually in the genital area. Secondary syphilis causes a rash on the body and mucous membranes. Latent syphilis has no symptoms but can be detected through blood tests. Tertiary syphilis can damage internal organs if left untreated.