Illness in a cat is not always easy to spot because cats often try to act as if they are not ill. This goes back to their more primitive days when a sick-looking cat would make an easy target for predators. For its own protection, a cat will try to look healthy, or otherwise hide away. This is why a normally sociable cat can become withdrawn when it is unwell.
Should you notice any of the following symptoms (besides any obvious wounds or broken limbs, etcetera), your cat is probably ill and needs urgent medical attention. Use this handy checklist as a guide: