Final answer:
Syphilis, a sexually transmitted infection, is caused by the spirochete Treponema pallidum, which leads to the development of a chancre.
Step-by-step explanation:
Syphilis, a sexually transmitted infection, is caused by the gram-negative spirochete Treponema pallidum. This spirochete is responsible for causing primary syphilis, which is characterized by the development of a chancre - a painless, ulcerated sore usually found on the genitals. The chancre is the first stage of syphilis and is associated with the initial infection.