Which of the following is caused by a virus?

Influenza is caused by a virus, specifically the influenza virus, which is known for causing respiratory infections in various species, including humans and animals. This viral pathogen can mutate frequently, leading to different strains which can cause seasonal flu outbreaks or even pandemics. It's important to clarify that while both salmonella and anaplasmosis are associated with infectious diseases, they are caused by bacteria—Salmonella is a genus of bacteria that can cause gastrointestinal illness, while anaplasmosis is caused by bacteria from the genus Anaplasma. Encephalitis, on the other hand, can be caused by various infectious agents, including viruses; however, in the context of the question, influenza is the clear choice as a specific viral infection typically referred to.