"Influenza" is probably just a catch-all term for common yearly respiratory illnesses (rhinovirus, adenovirus, coronavirus, influenza virus, hay fever, etc) and their symptoms. Check out what viruses are used in older vaccines.

Person: "I think I have the flu, I'm feeling very sick"

Doctor: "Yes it appears you do"

Data Scientist: Chalk up another for flu!

They're not actually testing everyone for what actual virus is present. They just assume it's an annual flu strain and look no further, unless it's dangerous, then it gets a Name and a full media/political campaign.