“Can I fly with the flu?”

If you are a pilot/flight crew definitely not. Illness will impact your cognitive abilities, focus, and stamina. Also, any sinus congestion will set you up for a sinus or ear block on descent. Pulminary congestion will make you more subject to hypoxia.

If you are a passenger, besides spreading the illness to others, you will have an awful flight experience. Would not recommend it.

The first three days of the flu I would not fly period you run the risk of contamination of the airspace around you. The first three days are the active contagion phase of the viral arc. Usually the fourth and fifth days of the arc you are not contagious.