Is Your Cat Afraid Of You?
Cats are non-confrontational unless cornered. This means a frightened cat will naturally try first to avoid whoever or whatever it is afraid of. A traumatised or shy cat will hide and try to leave the area, and when it has to be around the feared object or person, it will get low to the ground in order to be harder to catch. Cats that do this should be respected and basically ignored so that they can observe the source of fear from a safe-feeling perspective and gradually learn that they are not in danger. And of course, some things are frightening for good reason and will always instinctively cause them to flee.
A cat that is given no way to escape a fear-inducing stimulus may lash out in self defense with a scratch or a bite. They might growl, hiss and/or arch up first, or it might happen without warning. Cats should never be cornered or forced as this will only emphasise the fear that they feel and associate with the source. If your cat doesn't want to be held or picked up, definitely don't forcibly restrain it because it just makes you seem more like a predator. Similarly, if the cat wants to live under the bed, allow it to have the security of that hiding place.
When a cat trusts you and wants comfort and closeness to you, it will come to you. This may take patience and frustration on your part, because you have earned to that trust. The anxiety/panic level has to come down, and this doesn't happen overnight despite your best intentions. Keep your voice soothing and conversational, don't make any sudden moves, absolutely never ever put your hands on your fragile and tiny cat in anger, and strive to remain a consistent source of predictable kindness. Be satisfied knowing that the cat is observing you and collecting data about whether you are safe to be around, and so be sure you look trustworthy.
Some cats unfortunately are so traumatised by past abuse that they never really relax — they remain spooky and quick to hide. If this is the case, do be understanding and gentle just as you would with a terrified person — no grabbing or overpowering; give them some space, and hope for the best.
Try not to take it personally. Remember what it is like to experience a serious scare, and then imagine that scare came from someone who was not your same species, did not speak your language and was ten times your size.
Always keep easily startled cats away from their natural enemies, like unfamiliar dogs and screaming children, for everybody's sake. Don't sneak up on a peacefully sleeping cat — be kind and let it have its rest. You don't know what it may have been through :)