When you maintain a fever,your nose is stuffed and your headache is spreading to your toes, your body is telling you to stay domestic in bed. Feeling sick is an evolutionary adaptation according to a hypothesis put forward by Prof. Guy Shakhar of the Weizmann Institute's Immunology Department and Dr. Keren Shakhar of the Psychology Department of the College of Management Academic Studies, and in a recent paper published in PLoS Biology.
Source: phys.org