All types of wild animals do their utmost to hide outward signs of sickness. Predators single out animals that appear weak and unable to defend themselves, so it is always in an animal's best interest to appear healthy, even when it is not. This particular fact often makes it difficult for you to detect a pet's illness until an advanced stage, at which point it will be harder to cure. It is therefore important to understand the normal behaviors of each of your pets, and to recognize changes that might indicate a problem.