Rabies is a fatal infection that is contagious to all warm-blooded animals, including people. It’s caused by a virus that affects the nervous system, producing symptoms such as fever, seizures, paralysis, slack jaw, inability to swallow, excessive salivation (hence the trademark foaming at the mouth), lack of coordination, unusual aggression or shyness, and other awful things. Rabies is still active throughout North America, making vaccination a must for dogs, as well as required by law in many areas.
Dogs (and other pets or domestic animals—cats, horses, cattle, etc.) are most often exposed through contact with wild animals. Bats, foxes, mongooses, skunks, raccoons, and coyotes are the most common forms of wildlife infected with rabies. In rare cases, vaccinated animals can become infected, so if your dog has been in a fight with a wild, warm-blooded animal of any species (cold-blooded animals cannot contract rabies), take him to the vet for preventive care right away.