CDC, Centers for Disease Control and Prevention Similarities and Differences between Flu and COVID-19 What is the difference between Influenza (Flu) and COVID-19? Influenza (flu) and COVID-19 are both contagious respiratory illnesses, but they are caused by different viruses. COVID-19 is caused by infection with a coronavirus first identified in 2019, and flu is caused by infection with influenza viruses. COVID-19 seems to spread more easily than the flu. However, as more people become fully ...