It really depends on what you have.
I've gotten really bad stomach sickness from eating straight up junk dry food and I drank giant refills of vitamin water from the gas station. Cheap Electrolytes. Tea, Gatorade, lemon water is all good for anything needing to flush your system advisable for most illness. Basically the same as Pedialyte which is way too expensive.
For viral and bacterial I'd go with what
@TravelinMags said. Crushed garlic (let oxidize ten minutes) echanecea augustafolia root (not a $10 bottle of pills made with leaves) made into tea or alcohol extract. Oregano oil taken a few drops in water which is really good for your heart and circulatory system. Can also be used for fungi infections (be sure to dilute used topically)
Turmeric root is a great anti inflammatory.
I've also used green or bentonite clay put in water and drank for allergy exposure. Zinc supplement can work wonders until you build an immunity to it.
Overall if your sick you should rest as much as possible and eat healthy foods like watery vegetables and antioxidants. Low sodium soup or broth is a good option for something a bit more filling.
The vitamins and minerals that come from bones in soup stock (chicken soup) has forever been used to treat illness.