Answer:
Enzymes are temperature specific they do not work under high temperature and they do not work even in a very low temperature.
They are denatured in high temperature as enzymes are proteins in nature. They no longer work under such high temperatures.
During a high fever the temperature of the body rises and and the enzymes in the body loose their original shape and so they do not fit in the substrate and therefore is no longer functional.
The enzymes no longer catalyses the reaction so high fever is dangerous for the body.