Syphilis, a sexually transmitted infection caused by the bacterium Treponema pallidum, has many synonyms that have been used throughout history. One of the most common is "the pox," which was used during the 16th and 17th centuries. Other synonyms used during this time included "the French disease" and "great pox." In the 18th and 19th centuries, the term "lues" was commonly used, and in the 20th century, "VD" (venereal disease) and "STD" (sexually transmitted disease) became more widely used. Today, syphilis is commonly referred to as "the great imitator," because its symptoms can mimic many other diseases, and it is often misdiagnosed.