The correct answer is Herpes simplex virus because it is caused by a virus, specifically the Herpes Simplex Virus (HSV) types 1 and 2. The other options—Chlamydia, Gonorrhea, and Syphilis—are caused by bacterial infections. HSV is characterized by the formation of painful vesicular lesions and has a tendency to cause recurrent outbreaks due to viral latency in nerve ganglia. It is important to distinguish viral STDs like HSV from bacterial ones, as the treatment for viral infections is primarily supportive and antiviral, whereas bacterial infections require antibiotics.

Reference: Harrison's Principles of Internal Medicine, J. Larry Jameson, 20th Edition.