text: Fever begins by a resetting of a "set point" which is a term used in physiology for where the body maintains its homeostasis.  So instead of your body trying to maintain your temperature at normal, say 98.6 degrees, it instead tries to maintain it at 102.  A fever fights pathogens because some pathogens like Rhinovirus (the common cold) prefer to replicate at a specific temperature.  Rhinovirus prefers a slightly colder than human body temperature to replicate and thats why it affects the upper respiratory tract (where it is slightly cooler).  If the temperature goes up, it doesn't replicate quite as well and the body has an easier time fighting it off.