Bone broth(preferably organic) is good when sick. Fruit juice in moderation can help keep the blood glucose levels up during a fever. Apple Cider Vinegar (I prefer Braggs with the Mother) 2-3 times a day can help boost the immune system. I take mine with honey in a small glass of water, however apple juice helps cut the vinegar taste. Vitamins C and D, but I think you said you were already doing that.
In my house, we use Young Living essential oils. Everyone gets a drop of Thieves each morning and evening to boost the immune system, and more when sick. When we have company or someone in the house is sick we diffuse Thieves all day long. It kills the germs and viruses in the air and helps keep it from spreading. When someone is coming down with something, or really sick, I use the heavy hitters Oregano and Thyme. We end up smelling like a pizza parlor, but I've been able to knock out colds in 24 hours.