Well, since you are sick I guess you don't have a lot of appetite and I can remember being sick and even eating chicken soup was hard for me because at that time I had no appetite at all but that was before I started working out. So I was sick maybe 1 month ago and what I did was I tried to eat a lot of meals maybe like 6-7 per day but small ones which was a lot of meals for me personally because I don't know for you guys but I hit my calorie intake eating 3-4 big meals instead of 6-8 small ones. So yeah I suggest that you try eating often and even if you don't feel like eating, try eating at least a little more because except for building muscle, your body now needs energy to fight the sickness and also remember to drink a lot of water to stay hydrated ! Hope you get better as soon as possible ! :)