Fever is the common term for pyrexia.
Many diseases can cause a fever, or elevated body temperature.

Normal (rectal) body temperature range for the dog is approximately 102 F (~38.9C)
Normal (rectal) body temperature range for the cat is approximately 101.5 F (~38.6C)

For more on Cat Diseases and Conditions, please see:
Cat Health - A to Z

For more on Dog Diseases and Conditions, please see:
Dog Health - A to Z

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*Values are +/- 1 degree F and +/- 0.5 degree C
from Merck Veterinary Manual, 8th ed.