whenever I get the flu or some type of fever I eat normally.
however if I am having the chills or something like that, I will drink a whole water bottle (500mL) and then go on about my day. Repeat this every 5 hours as needed, but drinking normally is fine after this.
I got a severe fever the other day, but I chugged a water bottle, played on the computer for a bit and 4 hours later I felt back to normal with just a minor sore throat.
If you are having trouble swallowing food because of a sore throat then soup is probably the best option.