Driving when you have the flu is not recommended especially if you feel fatigued and have a loss of concentration.
If the flu isn't too severe yet then driving with the flu is usually safe but when the flu has gotten too severe where your body is weak, you have fatigue and cannot concentrate properly then it would not be a good idea to drive while you have the flu.
I have drove when I had the flu and I was okay but I too less traveled roads where I could drive slower and stayed off main highways and freeways.
But if I felt too fatigued from the flu then I would not drive because you could get into a car accident and if you drive while having the flu and cause a wreck due to the flu then you could be held liable for anything that happens.