Option 1: Bird-flu: Bird flu, also known as avian influenza, is indeed a disease caused by a virus. It primarily affects birds but can also be transmitted to humans.

Option 2: Chicken pox: Chicken pox is caused by the varicella-zoster virus. It is highly contagious and primarily affects children. However, it can also affect adults who have not been previously exposed to the virus.

Option 3: Cholera: Cholera is not caused by a virus, but rather by the bacterium Vibrio cholerae. It is an infectious disease that leads to severe diarrhea and dehydration.

Option 4: Dengue: Dengue is caused by the dengue virus, which is transmitted through the bite of infected mosquitoes. It is a tropical disease that can result in severe flu-like symptoms and, in some cases, can be life-threatening.

Based on the options provided, option 3, cholera, is not a disease caused by a virus. It is caused by the bacterium Vibrio cholerae. Therefore, the correct answer is option 3.