Flu can have mild to severe symptoms.
And I thought the Doctor's Gold Standard test for flu is a viral culture.
Come on now, this is a thread about flu on MN, get out of here with your facts! 
When I had medically confirmed H1N1 (swine flu) I was able to post online, I didn't have to crawl to get to the toilet, and I wasn't hallucinating. I was definitely too ill to work though.
I go in with a bad cold, have been in with tonsillitis, migraines etc but would have time off for a sickness bug or actual flu. Everywhere I've ever worked there has been a strict and rigourously enforced sickness policy that meant taking sick leave every time you have a cold isn't an option if you want to keep your job.