Which of the following sexually transmitted disease is not caused by a virus ?

The correct option is Gonorrhoea.

To understand why this is the correct answer, let's break down the causative agents of each of the sexually transmitted diseases (STDs) mentioned in the options:
1. Genital herpes: This is a viral infection caused by the Herpes Simplex Virus (specifically HSV-1 or HSV-2).
2. Gonorrhoea: This is a bacterial infection caused by the bacterium Neisseria gonorrhoeae.
3. Genital warts: This is a viral infection caused by certain strains of the Human Papillomavirus (HPV).
4. Hepatitis-B: This is a viral infection of the liver caused by the Hepatitis B Virus (HBV).

Since Genital herpes, Genital warts, and Hepatitis-B are all viral infections, and Gonorrhoea is a bacterial infection, Gonorrhoea is the only disease in the list that is not caused by a virus.