This article is from the Canine Medical Information FAQ, by Cindy Tittle Moore with numerous contributions by others.
(summarized From Carlson & Giffin)
Distemper is the leading cause of infectious disease death in dogs,
most commonly in unvaccinated puppies 3-8 months of age. Among
infected dogs: half show little in the way of illness; many show mild
symptoms; and in a few the illness is severe or fatal. Malnourished
and ill-kept dogs tend to show more acute forms of the disease.
Secondary infections and complications with distemper are common.
Prognosis depends on how quickly the dog is diagnosed and treated, and
which form of the disease the dog has.
There are two stages. Symptoms in the first stage include fever, loss
of appetite, listlessness, and a watery discharge from the eyes and
nose. It may appear like a cold -- but dogs do not get colds the way
people do; a "cold" is therefore much more serious in a dog than in a
person. Within a few days the discharge will thicken: a primary
indication of distemper. Dry cough, pus blisters on the stomach,
diarrhea (and associated dehydration) may follow. At this point, the
dog may recover, or proceed on to the second stage which involves the
brain. Dogs with brain involvement do not usually survive.