Once my one cat Tinker got attacked by a cat after he accidentally got out of the house. His back was shaved due to the fight. We noticed him feeling feverish, and called the vet, and made an appointment. The vet said he had a fever, and he said he probably wouldn't have made it if we wouldn't have brought him in that day. I'll always take my cats to the vet if they got into a fight, and looked even a little ill from it.