Syphilis is a common but serious sexually transmitted disease (STD) caused by the bacterium, Treponema pallidum. When a person has direct physical contact with another person who has a syphilitic sore or lesion, more commonly known as a chancre, there is a likelihood of transmission. Initially painless and circular, chancres can be found on or around the external genitals, in the vagina, or around the anus. They can also appear in the rectum and in or around the mouth.