Primary syphilis is the first stage of the sexually transmitted infection caused by the bacterium Treponema pallidum. It typically begins with the appearance of a painless sore, known as a chancre, at the site where the bacteria entered the body, usually on the genitals, anus, or mouth. This sore usually heals on its own within a few weeks, but the infection remains in the body. If left untreated, primary syphilis can progress to secondary syphilis, which involves more widespread symptoms. Early diagnosis and treatment with antibiotics, such as penicillin, are crucial to prevent complications and further transmission of the infection.