depends on what kind of sickness you have. for example, you shouldn't eat ice cream when u have a cold, but ice cream is good for soothing your sore throat. but you shouldn't eat fruits, juice, and sweats when u have a sore throat. this makes your throat drier. you should also never eat scratchy foods such as chips, nuts, and granola when u have a sore throat.
when your feeling nauseous, you should never eat greasy, spicy, oily foods, caffeine, alcohol, and carbonated drinks because these foods can make your nauseousness worse.
so, it basically depends what kind of sickness you have because food one can help sickness one while it is bad for sickness number two.