Why does your appetite decrease when you get sick?

When a person gets sick, especially from an infectious agent like a bacterium, the body mounts a vigorous inflammatory response, said Dr. Clifford Saper, chief of neurology at Beth Israel Deaconess Medical Center. Among other things, the immune system pumps out hormones that end up in the brain, triggering fever, changes in the sleep-wake cycle (that’s why you get so sleepy when you get sick), and appetite.