Pain Related Aggression - First and foremost you must see a vet. If your cat has suddenly become aggressive for what appears to be no reason at all then you must first rule out any medical problems. An abscess or wound from a fight may not be visible to us but can cause the cat extreme pain. There could be any number of reasons why a cat is in pain so you MUST get the cat checked by a vet first to rule out any medical reason before looking at the situation from a behavioural point of view.
Fear Related Aggression - the period from 2- 8 weeks of age in a kitten’s life is very important. They should be handled for at least one hour a day and should see at least 8 different people during this period. They should then grow up to be comfortable with being handled by humans. Kittens that are not well socialised learn to fear humans and will be defensively aggressive towards us. Kittens soon learn that if they hiss and spit then we back off - in the end the behaviour is done automatically as soon as a human approaches.