Understand the nature of sexually transmitted diseases (STDs) and recognize that they can be caused by different types of pathogens, including bacteria, viruses, and parasites.
Identify the pathogens responsible for each of the STDs listed in the problem: Chlamydia, Syphilis, Gonorrhea, and Genital herpes.
Chlamydia is caused by the bacterium *Chlamydia trachomatis*. Syphilis is caused by the bacterium *Treponema pallidum*. Gonorrhea is caused by the bacterium *Neisseria gonorrhoeae*.
Genital herpes is caused by the herpes simplex virus (HSV), which is a viral pathogen. This distinguishes it from the other STDs listed, which are bacterial.
Conclude that among the options provided, Genital herpes is the STD caused by a virus, specifically the herpes simplex virus.