Fever is the temporary increase in the body’s temperature, in response to some disease or illness. Fever is an important part of the body’s defence against infection. Rising body temperatures apparently are capable of killing off many disease- producing organisms. Many infants and children develop high fevers with minor viral illnesses. An adult probably has a fever when their temperature is above 99 – 99.5 °F (37.2 – 37.5 °C), depending on what time of the day it is.