I have never seen a case of typical distemper but it can cause brain damage and seizures and be hard to diagnose in adult dogs. The virus that causes measles in humans is similar to the distemper virus. In addition to the typical rash, measles can cause pneumonia, brain damage, seizures, and 1 or 2 children out of thousand who get the disease will die. Now we see only a few cases a year. We now vaccinate for feline leukemia and the incidence has decreased dramatically.
Most cases occur in unvaccinated feral and stray cats.
Tetanus shots in people and a widespread rabies vaccination program in dogs and cats have eliminated these diseases - almost. They are both still out there and without continued vaccinations, they could both flare up again. Research and development of vaccinations has continued and intensified in recent years. Improvements have been made and more are coming. The benefits have increased and the risks have diminished.