Puppy with distemper with pussy nasal discharge, by yourownvet.com
The D stands for canine distemper, an often fatal viral disease that suppresses the immune system. In addition to nonspecific signs of illness, such as fever and depression, a dog with distemper will often show signs of a respiratory infection, such as coughing and a discharge from the nose and eyes.
The distemper virus can affect multiple body systems, including the nervous system, leading to neurological signs such as convulsions or seizures. Most dogs whose nervous systems are affected will eventually die of the disease. Those who survive will likely experience permanent neurological problems.