Satiety and hunger are controlled by hormonal circuits. If something interferes with the ability to release those hormones, or the bodies ability to respond to those hormones, then they will be impacted. See circuits for Ghrelin, Leptin. For example if an autoimmune disease targets cells holding ghrelin receptors in the hypothalamus then you might get anorexia (as well as brain damage).

A major thing that goes on during viral infections is the bodies release of interferon, which has the side effect of reducing hunger. https://pubmed.ncbi.nlm.nih.gov/?term=interferon+hunger&filter=pubt.review