A chancre is a painless sore that typically appears as the first sign of syphilis, a sexually transmitted infection caused by the bacterium Treponema pallidum. It usually develops at the site where the bacteria entered the body, often on the genitals, anus, or mouth. The sore is usually round, firm, and can last for several weeks if left untreated. Chancres are an important indicator of primary syphilis, the initial stage of the infection. If not treated, the infection can progress to more severe stages, leading to serious health complications. Early detection and treatment with antibiotics, such as penicillin, are crucial for effective management.