Fact 1: Viruses cause many human diseases.

Fact 2: Influenza virus causes widespread human disease.

Given the two facts above, answer the question "What is influenza caused by?" with the following options: 
- insects
 - A bacteria
 - smoking
 - Mold
 - Viruses
 - amoeba
 - Cold weather
 - chemicals


 

     Viruses    