A person burns more calories when ill because the job of fighting disease requires additional energy, according to Scientific American. Eat well, but do not overeat when ill; the body is naturally an efficient machine for converting food to energy, and excess food to fat.
Eating helps the body to create more heat when a person has a cold. A fever causes an increase in the normal rate of metabolism, burning more calories. This is why getting enough calories is important for fighting off the illness, according to Scientific American. However, hydration is even more important than eating. Both fevers and colds drain the body of the water needed to combat illness.