No real difference, but you would be more likely to say "...a fever". (Just a thought... A: My boss is at home with fever. B: Oh really? Who's Fever?) 🙂

The second one sounds more formal... Maybe even antiquated but both are correct

Both are correct. My internal parsing is that "fever" can be both an uncountable noun for "the state of having an elevated body temperature," and both a countable noun for "an episode of fever." In your context, I think "a fever" is more natural, at least in US English. I am having trouble coming up with any rules or explanations of when we use each form. It seems to me that I would say "the symptoms of malaria are chills and fever," or that someone with a temperature 2° above normal "has two degrees of fever."

I agree with Charlie, there's no real difference. Although I'd say. The boss is at home and he has a fever.

OK! :-) Thank you!!!!