YOu need to keep your energy up but also make sure you get your vitamins in too; I agree that you should take a daily vitamin. And try to have some veggie or chicken noodle soup each day. You can also have diet or regular ginger ale and tea, which I drank a lot of during my last bad flu (my grammie's advice).
I think having a protein shake; either homemade or buy them if you are too sick to make one. You can buy them at your local drug store; and you can get those Carnation Instant breakfasts at the grocery store too. Don't forget that you can eat soft fruits like applesauce, pureed peaches or pears, and jello; I've even seen people eat baby jars of fruit when sick. Bananas and oranges to get in some potassium.
I really liked chicken noodle soup with a few whole grain soda crackers. You can get that in an instant cup-a-soup too (lipton) or the one you make in a pot. Later on, I would eat scrambled eggs for protein; and sometimes a slice of whole grain bread with peanut butter.
Hope you are feeling better soon ...