what's the difference between a really bad cold and influenza?

what’s the difference between a really bad cold and influenza?

I’m not seeking medical advice here - it’s just that I’ve occasionally heard someone say “so-and-so finally went to the doctor about that bad cold and the doc says it’s influenza.” The symptoms sound similar - fevers, congestion, etc. I understand that flu is more serious - but how does a doc tell the difference, short of testing the virus?

IIRC, “colds” as we know them are caused by rhinoviruses (“nose viruses”) and tend to affect the sinuses, nose, and upper respiratory tract, while real influenza is a disease of the lungs–lower and more serious than a cold.

The second paragraph of this article details the difference a little more.