Is stopping drinking alcohol during illness like the flu or bronchitis important or not really?

You should avoid alcohol. When you are sick, especially with bronchitis or the flu, your body needs fluids so you don't get dehyrdrated. Alcohol is a diuretic so it flushes out water, you want to retain water and flush out the bad stuff. It is also hard on your other systems in your body to fight your illness and process the alcohol.