Dogs suffering from diarrhea are not a worsening of the condition, but are normal clinical symptoms of the disease. Generally, dogs will have blood in their stools in the middle and late stages of the disease, but there is still great hope for cure, so they must be actively treated. . The parvovirus period is most serious when it lasts three to five days. If the dog can survive it, it will begin to slowly improve on the sixth or seventh day.