In US, the dog bite law consists of civil and criminal law found in state statutes, county and city ordinances and court decisions. These laws vary from state to state.
Eighteen states in the US observe the "one-bite rule" which shields a dog owner from liability for the first bite injury to a human, unless the owner knew the dog tended to bite, caused the bite negligently or intentionally, or violated a leash law or other animal control law. In short every dog gets a chance of one free killing or mauling of a human being and the owner is not held liable for the dog’s first attack. In the other states, a dog owner usually is liable for the first and all subsequent bites even if the dog previously was a "good dog." However there are important exceptions and limits to these general rules. Injuries caused by negligent handling or confinement of a dog, or by violating a leash law, make a dog owner liable in almost all states.