Yeah, flu is not typically a gastrointestinal illness but a respiratory one. It can be diagnosed by a physician without a test, but of course a test will confirm it.
State health departments collect information on flu cases - people who go to the ER or urgent care center with flu-like symptoms - and report it to the CDC. I remember hearing about that when I worked at the medical school. There was a really bad outbreak several years ago and we all had to be very careful, even though, in my office, we had no patient contact.
The trouble with eating Italian food is that five or six days later you're hungry again. ~ George Miller