Rabies is caused by a viral infection of the nervous system. Most cases of rabies in the United States occur in wild animals. Because dogs share territory with wild animals, they're at risk of being bitten by a rabid wild animal. (Normally timid animals can become aggressive if rabid.) Most cases of rabies in dogs can be traced to skunks, foxes, raccoons, and bats, but any rabid mammal can transmit the disease.
The risk of contracting rabies from your dog is extremely small, but the disease is so deadly that, if your dog were to contract it, he would need to be humanely killed, and you would need to have a series of inoculations for your own protection.
A dog with rabies may hide, become agitated or nervous, get weak in the hindquarters, or become aggressive. Swallowing difficulties are also common. Whether he's vaccinated or not, and if you see symptoms or not, if you suspect your dog has tangled with a wild animal, contact your veterinarian and local public health officials immediately. Your life may depend on it! If your dog is current on his vaccination, he'll need to be quarantined, but if not, public health officials may require that he be killed. That's because the only way to tell for certain that an animal is rabid is to test brain tissues.
Can you possibly need any other good reasons to vaccinate your dog against this deadly, contagious disease? Don't take a chance!