A "Virus"

What do you think constitutes a "virus". Sometimes I wonder if that's even a true sickness. I think it's a better way to say that they don't know what's wrong. LOL. My daughter has been sick for a week now. She has had a "virus". Her fever was 103.5!!! I was kind of scared. She's never had a fever that high! Anyway.......she is finally better but holy cow, it feels awful to just hear it's a virus.