A fever is a medical symptom that describes an abnormally high increase in body temperature. It is most often a sign of systemic infection or sepsis.
When Dr. Phlox visited Captain Jonathan Archer in his cell on Narendra III in 2152, Phlox pretended to diagnose Archer with xenopolycythemia in order to talk to him. Phlox asked Archer if he had any residual symptoms, such as a fever or dizziness, to which Archer replied that he only had a scratchy throat. (ENT: "Judgment")
Leonard McCoy ran a high fever as a bad side effect of an accidental drug overdose of cordrazine, thus becoming horribly delusional and weak. He was nursed back to health by Edith Keeler. (TOS: "The City on the Edge of Forever")
Spock suffered from fever, when he was undergoing plak tow, thus causing him some obvious physical discomfort and disorientation during his fight with James T. Kirk. (TOS: "Amok Time")
Captain Jonathan Archer suffered from fever after T'Pau performed a Vulcan ritual on him. (ENT: "Awakening")
A high fever was a symptom of the "unknown virus" epidemic that infected 1/3 of the imhabitants of the USS Enterprise-D in 2364. (TNG: "Angel One")
Kamin had been feverish for three days when Jean-Luc Picard awakened in his place. (TNG: "The Inner Light")
A high fever was a late symptom of the aphasia virus which Chief Miles O'Brien suffered from in 2369. (DS9: "Babel")
A fever was a symptom of the Larosian virus. (DS9: "If Wishes Were Horses")
Victims of the Harvester biogenic weapon would show a fever. (DS9: "Armageddon Game")
Fever was an early sign of the macrovirus. (VOY: "Macrocosm")
Vargas, a Starfleet officer stationed on AR-558, contracted a fever in 2375. (DS9: "The Siege of AR-558")