Final answer:
Syphilis, a serious STD caused by the bacterium Treponema pallidum, begins with a painless chancre and can lead to severe health problems if left untreated.
Step-by-step explanation:
The sexually transmitted disease (STD) caused by the bacterium Treponema pallidum is syphilis. This bacterial infection has several stages, with the primary stage presenting as a painless genital sore, known as a chancre. It has three stages: primary, secondary, and tertiary. Primary syphilis is associated with a painless hard chancre lesion on the genitalia. If untreated, syphilis can progress to more serious health issues, affecting the heart, brain, and other organs, and may eventually become fatal. Syphilis is less common than chlamydia or gonorrhea but it requires prompt treatment once diagnosed to avoid severe complications.