"A dog is entitled to one bite." I heard that the other day.
This is not true in Illinois. A dog that attacks or attempts to attack a person can cause its owner to be liable in money damages to anyone injured by the dog, even if the dog does not succeed in biting that person. So no, in Illinois, a dog does not get a free first bite. On the other hand, a dog has something like a self defense plea. If the victim provokes the dog, there is no claim.
Proving the dog is a good or a bad is no defense to a claim against the owner. The Animal Control Act provides that a dog owner is strictly responsible to the victim of a dog attack. No negligence by the dog owner need take place.
But more people than just the owner may be responsible in money damages: anyone in control of the dog, if negligent, may also be sued. That could include dog sitters, dog walkers, or just plain any person at the other end of the leash.
If you have been injured in an Illinois dog bite accident, contact a knowledgeable Hinsdale personal injury attorney.