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If your vet suspects Parvo, he can do a simple blood test that is not expensive and either rule it out or confirm it. If he has already done it, it's pretty reliable. You don't often get a false negative.
Parvo usually presents with a fever, lethargy, bloody diarrhea with a smell unlike anything you have ever smelled before and vomiting.
If you suspect parvo, you need to force fluids. Give pedialyte as often as you can get it in him. Syringe it in his mouth if you have to. The disease kills because it dehydrates the dog.
Since you are seeing improvements, I doubt that it parvo. I would lean more towards and infection because of the conjunctivitis. Most dogs require antibiotics for this.
I hope that I have answered all of your questions. If you have more or I have missed something, please let me know.
Distemper is just not a very common disease....
The clinical signs of AIHA are usually gradual and progressive, but occasionally an apparently healthy pet suddenly collapses in an acute hemolytic crisis. The signs are usually related to lack of oxygen: weakness, lethargy, anorexia, and an increase in the heart rate and respirations. Heart murmurs and pale mucous membranes (gums, eyelids, etc.) may also be present. More severe cases also have a fever and "icterus" (jaundice), a yellow discoloration of the gums, eyes, and skin. This is due to a buildup of bilirubin, one of the breakdown products of hemoglobin. (source)
I really feel led to think that this has more to do with an infection.