Fever is not a tier 3 symptom, and a tier 2 symptom in some other diseases, such as the Simian Flu DLC and the Necroa Virus DLC. It increases body temperature, contagiousness, and severe dehydration through excessive sweating. It slightly increases disease lethality and Infectivity, although it is more lethal than Paralysis.
In Necroa Virus, Fever, along with Coma, weren't lethal. They do have other effects. In Necroa Virus, while Coma reduces infectivity, Fever slows research speed.
Along with Diarrhoea, Tumours and Inflammation, Fever is one of four symptoms which can be better understood by scientists during a game, making a plague easier to cure when it is evolved or mutated.