Which one of the following is caused by a virus?

Influenza is an infectious respiratory illness caused by influenza viruses infecting the nose, throat and lungs.

Hansen's disease (commonly known as leprosy) is an infection caused by Mycobacterium leprae, a slow-growing bacteria.

Cholera is an acute diarrhoeal illness caused by the Vibrio cholerae bacteria infecting the intestine.

Malaria is caused by the Plasmodium parasite, which is a single-celled parasite. Mosquito bites are the most prevalent way for the parasite to infect humans.