The correct answer is option 1: Influenza. Influenza, also known as the flu, is caused by a virus. Influenza viruses infect the respiratory tract, leading to symptoms like fever, body aches, cough, and fatigue.
Option 2: Pneumonia is not caused by a virus. Pneumonia is an infection that can be caused by bacteria, viruses, or fungi.
Option 3: Cholera is not caused by a virus. Cholera is a bacterial infection caused by the bacterium Vibrio cholerae.
Option 4: Whooping cough, also known as pertussis, is caused by the bacterium Bordetella pertussis, not a virus.
In summary, out of the given options, only influenza is caused by a virus.
Alert - correct answer should be 2: Pneumonia is caused by viruses, bacteria, or fungi.