That sounds like Parvovirus, possibly coronavirus. Yes, I've had a dog that had it, the puppy barely survived it. It's extremely quick acting and some breeds of dogs like rotts are more succeptible to it than others. It's preventable by a vaccination usually. Starting symptoms loss of appetite, diarrhea. Sometimes vomitting. When the dog's diarrhea starts looking bloody that means if you don't get it treated immediately the dog can die within 24 hours.