When people suffer from a number of different illnesses, their bodies release chemicals known as cytokines as part of a larger inflammatory response. One of the effects that the cytokines have on the body is a reduction in appetite. In some illnesses, altered hormone levels also reduce the appetite.
One reason why the appetite slows down may be to set aside some energy to allow the body to fight disease with the immune system. While eating contributes energy, digesting the food consumes energy, and when the body does not have to digest food, it has more energy to fight off illness. While this can be a positive effect in conditions that do not last a long time, in diseases that last a longer time, as with some forms of cancer, this reduction in appetite often leads to weight loss that is unhealthy.
Some diseases impair taste and smell, including versions of the common cold. When favorite foods do not taste as good, appetite tends to decrease as well. Finally, when appetite decreases, some viruses lack what they need for further growth. Certain bacteria require iron and glucose in the blood for sustenance, and cutting down on intake impairs their growth as well.