Is it ensure, insure, or assure?
late Old English fefor, fefer "fever," from Latin febris "fever," related to fovere "to warm, heat," probably from PIE root *dhegh- "burn" (cf. Gothic dags, Old English dæg "day," originally "the heat"); but some suggest a reduplication of a root represented by Sanskrit *bhur- "to be restless."
Adopted into most Germanic languages (cf. German Fieber, Swedish feber, Danish feber), but not in Dutch. English spelling influenced by Old French fievre. Replaced Old English hriðing. Extended sense of "intense nervous excitement" is from 1580s.
fever fe·ver (fē'vər)
Body temperature above the normal of 98.6°F (37°C). Also called pyrexia.
Any of various diseases in which there is an elevation of the body temperature above normal.
A body temperature that is higher than normal. Fever is the body's natural response to the release of substances called pyrogens by infectious agents such as bacteria and viruses. The pyrogens stimulate the hypothalamus in the brain to conserve heat and increase the basal metabolic rate.
(Deut. 28:22; Matt. 8:14; Mark 1:30; John 4:52; Acts 28:8), a burning heat, as the word so rendered denotes, which attends all febrile attacks. In all Eastern countries such diseases are very common. Peter's wife's mother is said to have suffered from a "great fever" (Luke 4:38), an instance of Luke's professional exactitude in describing disease. He adopts here the technical medical distinction, as in those times fevers were divided into the "great" and the "less."