Canine Distemper:

This disease, caused by the morbillivirus (a paramyxovirus), causes respiratory, digestive, and nervous system signs in affected dogs, and can be fatal in about half of unvaccinated dogs. Recovered dogs may have permanent damage to their nervous systems. The chronic form of the disease can lead to hard pad disease, a chronic thickening of foot pads, and encephalitis, an inflammation of the brain. Some dogs that acquire the virus show no signs or very mild signs, but can easily infect other susceptible dogs. Unvaccinated dogs are at a 350 fold increased risk of contracting this highly contagious disease which is spread by discharges from the nose and eyes of infected dogs.

The above description is provided by the CVMA (Canadian Veterinary Medical Association)